Just for the record: it’s still possible to get a free S/MIME cert nowadays e.g. https://www.actalis.com/s-mime-certificates — not affiliated, but checked it today and got a valid one, it’s still a bit of a hassle clicking through the forms there :-/
If it would be more convenient I guess regular people wouldn’t mind it being centralized. The same way as domain TLS certificate authorities operate now.
As for PGP, the current “schism” where GnuPG forked OpenPGP into their own, proprietary https://librepgp.org/ won’t help the interoperability, I’m afraid :(