The difference between what you've built and TrueNAS may well only become evident if your ZFS becomes corrupted in the future. That isn't to say YOU won't be able to fix it in the future, but I wouldn't assume that the average TrueNAS user could.

You... swap the drive, run a ZFS replace command, and that's it. I know I'm coming at this from a particular perspective, but what am I missing?

I believe the Olympics is meant to be a career for the rich, and the IOC will do its best to keep working-class people out by removing their opportunities, especially in popular sports. If too many rich kids can't get into Olympic teams, then the IOC will make sure that avenues like OnlyFans are banned.