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.