Email works reasonably well because a) we can afford to burn computational resources, b) spamming is not very lucrative, and c) we can hide the impact from end users.
Looking at my mail logs, 96% of recent mail on my server is spam. Sampling what I get, it's mostly incredibly dumb-ass garbage, built to take mostly small amounts of money from very unsophisticated people. Nobody is getting rich off it, partly because spamming is illegal and they will put you in jail [1] for it if you do it at large enough scales to make real money. Most importantly, end users don't see the spam, so they don't even know how terrible the medium is.
For financial markets, the way it gets solved in the real world is mostly by investors moving to better markets, ones that are strongly regulated. And it's not just investors. Companies will list on highly regulated markets like the NYSE and the NASDAQ as shows of strength and as ways of tapping the more available, less expensive capital that comes from trust.
So my bet is that the market will solve it by driving entire marketplaces or kinds of assets out of business.
[1] e.g., https://www.theverge.com/2016/6/16/11952266/sanford-wallace-...
