I'm not talking about the short-term, not the medium-term, but the long-term.
1. 51% attack
2. Centralization of the coin due to various factors which undermines its purpose (it will continue to exist, it just won't be good anymore)
3. Ever growing chain length does not recognize the ending of moore's law.
1. less of an issue as time goes on. regardless, not much can be done with a 51% attack anyway.
2. not really an issue. every argument ive seen about centralization misses key points on how they are wrong.
3. Pruning.
(this has all been gone over a crazy number of times and people still post crap after proper answers. and people still never look for answers)
Having to use fiat to use bitcoin is probably enough of a reason to stop A LOT of people from having a desire to use bitcoin. I would assume a lot of people don't care about any of the downsides of fiat so long as it's working for them right now.
I'll give you an example. If someone was about to walk into a coffee shop with $5 to buy a cup of a coffee and you said "hey give me that $5 an ill give you $5 worth of bitcoin and you can buy your coffee with it" MOST people would have zero interest in that because they see no point in it and it's extra one extra step just to buy something they can already buy.
With that said, that's fine. Bitcoin doesn't have to be used by everyone nor would it probably even be practical to use in certain situations, but I do believe everything I just said is a big reason why people would or do have no interest in bitcoin. It just seems pointless to them.
Even if someones fiat currency crashed, they would still need to use it to use bitcoin if they didn't have any bitcoin. So again they would ask the question "why use bitcoin when I can already use the fiat currency I have".
i think one basic counter point to this is that no matter what, the gov waters down our USD. our money loses value. bitcoin may have its ups and downs currently but over the course of 6 months to a year, no matter what is happening, the bitcoins you hold are worth more now then they were before even if you included buying on the top of a 'crash'. If you have money to put into bitcoin that you can leave there then there really isnt a reason not to. having the ability to easily access that money and buy things with it is also really nice. once in the bitcoin system you dont have all those pesky bank and 3rd party fees. it may tie cash for ease of use. but eventually we may see some really interesting security features t protect your bitcoins from in person theft as well as online theft. really its only a matter of time before such things are history where it will be like "wait you used to have your wallet on your PC? no hardware encryption, multi sig or dedicated wallet system? dam i cant imagine risking half a bitcoin! that would have been my salary for year!"