Based on my personal experience providing support for s/mime setups, you'd need: 1) A centralised solution for managing keys and certificates, connected to a login that the customer will be able to recover in pretty much any situation. 2) Email client support for fetching keys/certificates from the centralised solution. 3) A massive focus on usability and end-user support, because most email users have no idea what a certificate is, or how to use it.

Denmark actually has something similar to this (Sikker mail) but it's mainly aimed at businesses. Based on what I've seen, this resulted in a market for services that bypass the E2E aspect of it because business users can't figure out how to use it. It is also noteworthy that despite this s/mime being available for everyone, Denmark has a public digital mailbox for all citizens and businesses in order to ensure availability.

S/mime is great. It is also not suited for people who barelly know what it is.