If Bitcoin solves the issues with scalability, and miners can indeed live off transaction fees, I don't see why there would be strong demand for altcoins. They are slowly dying already, and maybe some will survive, but I don't think a big number of altcoins will be in use. By 2140 a lot of stuff can happen. Humanity might finally kill itself and the survivors won't have time to deal with cryptos; quantum computing might see a breakthrough with Bitcoin and most cryptos rapidly failing (but new quantum-resistant ones probably taking their place). I think the fact that all BTC has been mined will be the least of concerns in any case because even if everything remains more or less fine with the world, prior to 2140 there'll be a long period of time with very small amounts of BTC mined and most miners probably already living off the transaction fees. So the way I imagine it, it won't be a rapid change.
Maybe we'll end up with a very small number of altcoins (mostly Ethereum, Litecoin, and other coins in the top 5 ranks in market cap) on the market by 2140. Bitcoin will still be relevant as a digital store of value anyone can use in times of need. My guess is that Bitcoin will be scalable enough for the whole mainstream world to use. Miners can still live off transaction fees, even when all coins are mined by 2140. With sidechains and many other useful features coming along the way, I believe that Bitcoin will become an "All-in-one" cryptocurrency in the future.
Of course, technology constantly evolves over time. If a new contender comes out with better tech than Blockchain, then Bitcoin won't be able to live that far. All of this is pure speculation than anything else. At least, we won't have to worry about Bitcoin's future in our lifetimes. We can continue to enjoy Bitcoin as is, leaving it as heritage to future generations. If Bitcoin continues to prioritize decentralization above anything else, it'll be able to live for a very long time. With the rapid development of the Blockchain industry, it's expected that Bitcoin solves scalability & privacy issues by 2140. Let's just focus on Bitcoin's development for the next 10 - 15 years. Just my opinion
