Hard to say if bitcoin will lose its official number 1 position. Of course, it will lose more and more market share, I think that's inevitable. However, you can still be market leader with 0.1% of market share, if there are hundreds of thousands of competitors.
If bitcoin would have stayed alone, it would have been a dumb greater-fool game (as was already seen in 2010 by some here). It is just a token you buy for real money (you fool!) to sell it for more money to another, greater fool. Such things always end badly, when they run out of greater fools - but in the mean time, the lesser fools can hope to pump a lot of wealth out of the latest layer of downright idiots that pay for the whole shebang. The later you come to the party, the riskier of course.
I don't think that many people buy a bitcoin for $ X with the idea to use it later to buy stuff worth more or less $ X, or to cash out again and obtain $ X. By far most people buying bitcoin for $ X hope to sell it for $ 100 X or something, like the early birds in the game. Nobody's buying a coin at $20 000, if he's not hoping that it will go to $ 2 M or something. Well, he can settle for $400 000, true. He would be quite disappointed if he could only get rid of it for $ 50 000 ever.
It is quite obvious that the one buying at $20 000, can maybe hope for $ 2 M, but the guy buying for $ 2 M, is he really going to hope it will go to $ 200 M ? Seriously ? But if the guy buying for $ 2 M cannot hope for $ 200 M, why would he buy at $ 2 M in the first place ? And if nobody is going to buy at $ 2 M then, how can the guy at $ 20 000 dream of $ 2 M ? But then, nobody is ever going to buy at $ 20 000 ! Nevertheless, people bought, can you imagine, coins at $ 20 000 !!
This is because experiments have shown that most people think that other people are stupid. And greed can render people somewhat more stupid than usual. However, it is becoming very difficult. If you go to high prices, not only do you have to find REALLY stupid people, they also have to be VERY NUMEROUS, and on top of that, VERY RICH, because to sustain the needed flow of money which is huge, a lot of people need to pay a lot of money for the coins. Well, from a certain point onward, one runs out of them. From a certain price onward, to sustain the price, you'd need too many stupid and rich people during too long a period. There are a lot of them, true, but it is nevertheless a limited offer. So at a certain point, one runs out of them, and the whole shebang crashes down.
The story, however, is entirely different if you have a market of speculative assets, moreover, an open market where new assets are created all the time. In such a market, you can RE-USE money: it goes from one asset to another, it comes back, it jumps on yet another one, there's the growers and the fallers of the day.... such a speculative casino can run an unlimited time on a limited amount of money, because some day, you win, some day, you lose, and there are continuously new opportunities to win big (and to lose a lot). This is the kind of casino that most financial institutions are used to, but where they are constrained by legal limits, to refrain from funny games ; here, everything goes. You go from Judo to Birman boxing. It's way more exciting.
So is bitcoin going to remain the reserve currency of this casino even if its market share will dwindle ? I don't know. But alt coins are what is the hope of survival of bitcoin.