...why would almost anyone using MongoDB care at all if it's included in a distro's set of default packages?! Don't we all always install software from repositories hosted by third parties? As long as the Mongo devs keep supporting RHEL, I'd see no problem for actual direct users of it. And SSPL sounds like a pretty good and sensible thing.
The whole point of using RHEL is that it has full support, why pay the premium for it if you then include non supported gears into it anyway.