I would think it more likely that Bitcoin evolves little by little, instead of being entirely replaced by an alt. coin.
Hopefully.
But some important innovations are architecture dependent and can not be done. It is at this point where Bitcoin is going to be replaced.
Please elaborate; I thought anything can be changed through a hard fork and which ever version has majority support wins out.
So I would think we'll start to see minor hard forks and each time it won't seem that big of a deal, but 10 years later you'll look back and think "holy crap bitcoin has changed a lot"
For example if I though that i had a much better idea for an alt. currency. I'd say that it would be better for me to make a private road map of how to make bitcoin into my new currency, and make small incremental pull requests over a long time period until i'd managed to change bitcoin into the better currency that i had envisioned; as opposed to trying to start my currency from scratch with zero public support.
Of course you might complain "But my pull requests or forks might get rejected"...well maybe that means that other people don't think that your ideas are better than the status quo, and starting your own alt. currency is only a way to postpone that realization. Unless of course you think the current devs are holding bitcoin back by rejecting your code, in which case you take your case to the message boards, if there is popular support for your modifications I'm sure the devs would be persuaded to include the code updates.
Yes very much can be changed with a hard-fork, but only up to a point.
Currently even something very minor from a software architecture standpoint like the block size limit, which is just a variable when it comes down to it generates massive controversy, and as we have seen bugs.
So I don't think that major changes are going to make it through.
The examples I posted above are things which could be much more easily implemented by starting from scratch. It's not entirely impossible to do this with a hard-fork but you also have to see this from the innovators perspective: Why try to convince the Bitcoin core team, and try to work with an old code-base instead of starting a own project, especially since the reward from doing so is so much higher?
In software development it only makes sense to do a new project once the amount of changes exceed a certain amount. This has been done over and over again and I can't see how cryptocurrencies would be any exception.
See Netscape/Firefox/Chrome, Apache/nginx, Gnu/*BSD.
If changes are sufficiently numerous people will create another software and this has nothing to do if it requires a hard-fork or not, it simply makes more sense.
Here's where your argument breaks down (in my opinion):
You seem to think that the Bitcoin vs. [as of yet unknown altcoin] situation can be compared to the browser history. Major innovations by a newcomer in this field more than once caused the previous market leader to lose its established position (Netscape replaced by IE, IE by Firefox, Firefox by Chrome).
However I hold that the *coin situation is more similar to the history of social networks. A major innovator, with sufficient momentum generated by word of mouth,
can sometimes replace the previous market leader -- say, when Facebook destroyed Myspace's entire business model. But innovation and word of mouth do not guarantee success in this field: upon release, Google+ had huge momentum, the whole might of the world's largest Internet company behind them, and was/is arguably a much more pleasant experience than Facebook. Yet, they completely tanked in their effort to even approach the position of Facebook.
The difference is previous investment in the product. Browsers require very little of it (just install a new one), social networks a lot (all your friends are on it, you'd have to convince them to follow you).
Now please be honest: in terms of investment, which of the two does a cryptocurrency resemble more? (caveat: I admit, there's always the possibility to shift your investment from one currency to another, and this is maybe somewhat easier than shifting "investements" in a social network. However, even with the option of trading currencies, this process is not frictionless, and will therefore be a serious barrier for any competitor)