You seem to be a reasonable man. Let's keep with the topic at hand and ask yourself the good questions.
Yes... much better in my thinking for us to delve into substantive issues, yet surely I am NO expert at all issues (especially complicated ones), even though I do have my opinions (views) regarding your various questions regarding the future of bitcoin.
Why is it that BTC growth rate increased by 8000%. According to the predictions, when there will be no more BTC available in 2124, what will happen to the growth rate?
From these two questions, I gather you are talking about two related issues... one issue is the appreciation in BTC prices and then the other issue is the decrease in the supply of BTC - over time and in the longer time future. I find it a little more easy to respond to them in the reverse order.... or in combination...
As you suggested, the growth of BTC supply is fairly extensive in the beginning - with the allowance of 50BTC blocks every 10 minutes for the first 4 years (2009 to 2012), and then 25 BTC blocks every 10 minutes the second 4 years (2013-2016) , and then halfing every 4 years thereafter, which will cause a large majority of the coins to be mined fairly quickly and early on (which I read as half of the coins are mined in the first 4 years.. then 25% in the next 4 years... etc etc)... So by the year about 2140 all of the blocks of coins will be mined.. but many more blocks are mined in the very beginning (front loaded).
There are a lot of complicated factors in describing how BTC prices have grown in the past 5 years, including increased adoption and speculation and investments... so as adoption of BTC grows and its network and its applications and its liquidity, the price will continue to grow at a fairly high rate (some call this anticipated exponential growth). Some have estimated (including forum members Slippery Slope and Peter R and maybe some others) that there will be something like 10x growth per year during the first half of the adoption phase of BTC... (surely this is bullish to the extreme) and depending on how wide-spread BTC becomes adopted, 1/2 of BTC's adoption phase could last for several years into the future... Currently probably less than 5% of the world's population have heard about BTC and less than 1% have considered investing in BTC and less than .01% have actually established some kind of holdings in BTC above .001BTC....... So there appears to be quite a lot of room for growth and adoption of BTC... in order to take us through the first half of its adoption phase.
Thereafter, the adoption phase would taper off and the price growth rate of BTC would also taper off. Maybe we do NOT believe in the exponential growth rate of 10X per year and instead we believe 5x or 2x or .5x or .1x or some other variation? Nonetheless, it is very likely that any particular growth rate will NOT be easy to predict or even that the growth rate may NOT even comply with any previous models regarding how a similar asset should grow. NO matter what the growth rate, the growth rate will affect price, both up and down... and there are so many people who are bullish about the growth of BTC based on these theories of growth... and some bears follow these theories and agree with them and some bears think the various exponential growth theories are rubbish.
In the end, there remains enough subunits of BTC (since BTC is divisible into 100 million sub-units (satoshis) per BTC) to supply the whole economic system with a small supply of BTC, and additionally, there could be a fork in the BTC blockchain that would allow for BTC to be divided into even smaller units.. if needed to supply the needed currency or storage of value or transmission of value or public ledger or other features.... BTC has several ways to flex to accommodate growth..
Your presentation of growth of BTC as a "problem" is very speculative into the way distance future or over problems that are NOT as wide-spread as you are making them out to be. Of course, these kinds of potential "problem" matters should be accounted for, but we really do NOT need to know the answers to all of these kinds of potential "problem" questions in order to know that BTC is a good value and a good investment today.. a good investment that may have some problems in the present that are being worked on and potential and anticipated problems in the future... but these various problems are NOT insurmountable (at least NOT insurmountable at the moment). Surely, there could be some unanticipated problems too... or some of the problems could become insurmountable... but to-date, the various listed and known problems are NOT insurmountable.
Regarding value: Any new and/or innovative system such as the internet, or the telephone network or certain companies such as microsoft and facebook, have experienced variations of exponential growth in their use and in their value and the transfer of wealth from one group of people to other people who had invested into the new businesses and/or the new paradigms.. same is true for BTC... BTC could be a winner or a loser, and in my humble bumble opinion, it is up to people to decide whether and how much to invest... and it seems better for those peeps who hear and learn about BTC earlier rather than later.
You can assume the doom and gloom of BTC all you want, and that is fine for you b/c you are correct that there are potential obstacles that need to be overcome (and some are mere speculations into a very distant future), yet a large number of the people participating in this forum, are invested, interested and informed about various crypto currencies.. including BTC.. but NOT necessarily locked into their various investments.
If a person believes that his/her investment into BTC is NOT paying off or is NOT panning-out as expected, then s/he can transfer his/her proceeds of his/her investment to some other asset class.... or even back into fiat, if s/he thinks that tranfering back to fiat or even into some other investment is the more prudent, safer or better course of action.
Again, even before those 12 million BTC left will be exhausted, guess what happens when quantum computing makes its debut? (not to mention the security issue, and the fact that is highly likely that it will be replaced by an altcoin that will have something more to offer, such as anonymat).
I think I have already addressed these kinds of issues with my above response, yet regarding quantum computing, we will need to see whether those kinds of technologies would necessarily undermine BTC's network of computer systems, its mining or its security.. all of which are continuing to be developed and built upon.
And, in the event that a superior alt coin comes along, then we can cross that bridge when we get there. At the moment, BTC has more than 95% of all of the crypto-currency market share, and BTC is the big gorilla and best game in town... If a competitor or two to BTC comes to be a challenge to BTC in which BTC is unable to absorb some of the features of the competitor alt(s), then investors can switch over to those kinds of coins.. b/c NO alt is going to take over all of BTC's market share in one swell swoop... absent some major technical glitch.. so, if a serious competitor comes along, then I would expect some NOTICE and signs..
In my opinion, BTC remains the best in the crypto currency world and likely among the better investments as a whole... of course with any investment, people need to consider their total investment portfolio and consider the extent to which they are adequately diversified. Personally, I have been putting a very large percentage of my new investment money into BTC - yet BTC occupies less than 10% of my total investment portfolio... even though it is a very large percentage of new investments that I make... b/c I consider BTC to be a GREAT current value (a great risk to value ratio).
Hopefully, this helps, even though a bit of a long answer.