I believe the difference between people who say "email self-hosting is dead" and people who say "email self-hosting is trivial" is probably the volume of mail sent from their domain(s).
At my busiest, I was sending dozens of emails per day, but they were all from my work account. My personal account is pretty close to recv-only and probably averages an outgoing message count in the single digits per week. How can I reasonably keep an IP/domain reputation score fresh/warm if I send such a low volume? The answer is I realistically can't.
Self-hosted email remains an extremely difficult and time-consuming endeavor unless you happen to have some good luck, it seems.