The current banking system is better right now, because it's at least usable and scalable. Of course the current banking system is centralized and expensive, which is a problem, but if a substantial portion of Bitcoin's userbase is arguing for that transaction speeds and costs are irrelevant because Bitcoin will be centralized too, then I'm not giving them credit for that.
And frankly, it's not at all true that no other coins have come around that are better. Monero is vastly better at privacy than Bitcoin in pretty much every way. There are already coins on the market that are using proof of stake today. In pretty much every area, the state of cryptocurrency has evolved over Bitcoin -- but, importantly, none of those coins have become better systems for speculative investing. And I would argue that's the "better enough" that most Bitcoin advocates care about.
I also don't see any reason to assume that once Bitcoin goes more mainstream that a community that is currently hostile to change because it might affect their wallets is suddenly going to be less hostile to change. Bitcoin could be evolving today. People could be switching to better coins today. But they're not. It's not going to be easier to make those improvements in the future.
Bitcoin is the technology. The idea that we need wide adoption of a technology that is ill suited for the problem it is trying to solve, just so that we can drop that technology and choose something that's actually usable on its own without first reimplementing the entire concept of banks -- it just doesn't make sense as a long-term strategy.
