I don't think they actually do have to spend it.
They could do what a lot of old money does, where they use it as collateral for loans, which they then use to grow their wealth, never actually selling, ever.
There is of course the interest to deal with, but that's not an insurmountable issue.
I also don't think Bitcoin is the one true solution that will kill all. There are many coins, with their own communities. Some of these are worthless, and others are worth a lot more than people think. Many fulfill a lot of the same uses that Bitcoin does, but Bitcoin remains king because it's the one everyone talks about.
If Bitcoin continues to be updated and improved, then it might be the king for a particular use case and investment. If we have a situation again like prior to the previous crash, where the costs to send Bitcoin are exorbitant and you can't even get your money, then we'll have some more hard times ahead.
There's also the "bubble" issue. It's not a bubble in the sense that crypto has very real use cases, but there is also a ton of money being thrown around causing shit coins to balloon. The bubble is not going to pop though, just like the dot com bubble never actually popped. They had a ton of stupid money flying around, and then everyone freaked out when reality started to set in, and many companies that should not have failed, did. Because the money pulled out. Yet here we are on the internet, which is bigger than ever. Crypto will continue to be big in the future. I think the way banks operate is more likely to go away with crypto.
I don't think it will just create new masters though, as crypto has also launched things like Hive and crypto efforts to create types of UBI. Eventually we'll figure out the exact way to crack this nut and ensure the old system no longer oppresses us. We will come to realize that it's not entitled to ask for food or help, but to expect to not give anything back to the community you are part of.
The entire system is self-correcting and littered with abundance. Someone thinks the other networks are being greedy: so they make a hopefully new and better one. They can even use the value generated in the new network to own more Bitcoin. It's all synergy. Very hard to compare to anything previous.