It seems like even if there are enough situations where the zero-trust nature proves to make sense to users, it would be silly for them to all use the same chains. Why deal with the overhead and transaction costs? Why wouldn't there be as many chains as their are email providers, and then most of them would only have value insomuch as the things running on them have value? So more like "new way of formalizing coalitions and joint ventures and partnerships where no single partner has full control" than "investment vehicle" or "foundational infrastructure for the whole world."
Email, by being a protocol vs a single implementation, has outlasted many email service providers and will probably outlast many more. Many of those service providers were killed by new entrants that could do it cheaper or better - so it seems crazy to expect otherwise-productive things to remain on these hideously expensive trendy chains of the moment.