I have thoughts on all this but no clear opinion on whether mythic raids should remain strictly 20-person.
My AOTC guild has occasionally made forays into mythic raids, just for the early bosses. But most of the time we don’t, and the strict 20-person raid size is the primary reason. Some tiers we’re a bit short and not interested in pugging for progression. Other tiers we have too many and have to leave some people out, and that really feels bad.
On top of that, we’re a “real life comes first” sort of guild designed for busy adults with families, demanding jobs, etc. We have no attendance requirements for our raids and do not pressure people to raid if they have real-life stuff going on. We find that this is not compatible with mythic raiding because of the fixed raid size.
Recently there was a reddit post from an expectant mother who said her husband was upset that their upcoming newborn’s delivery conflicted with his raid schedule. His guild was on mythic Jailer. That is awful on so many levels (on the husband’s part), but I understand the pressure he is feeling. That’s exactly the pressure my guild has decided to avoid by not raiding mythic.
The last time we raided mythic was in 8.3, where we killed 4 mythic bosses after finishing AOTC. In Shadowlands the desire to raid mythic just has not been present in my guild. For one, the raids feel harder, so by the time we get AOTC we’re ready for a break from raiding. But the main reason is that we’ve gotten heavily into mythic+ instead. Mythic+ offers all the difficulty of mythic raiding once the keys get high enough, but in smaller time chunks and in smaller, more flexible teams. We have a wide variety of skill levels in the guild, and mythic+ allows us to each run at our own level, whereas in raids there can be frustration from the more skilled players at having to wait for others to master the mechanics.
Mythic+ has basically replaced the early mythic bosses for us.
The one thing that has been frustrating for us has been that not raiding mythic has left us unable to get the best gear, which impacts our ability to do high keys. In 9.2, this was greatly ameliorated by the Creation Catalyst. Yeah, there are a few raid trinkets and weapons we still can’t get at high item levels. But the Creation Catalyst and itemization in general have allowed us to gear up to a place that is close enough to what mythic raiders can get that we can do keys higher than 20.
That said, I always do look a bit wistfully at the mythic encounters. I read a post, I think it was here, where the person said mythic Vigilant Guardian felt like a more complete encounter than the LFR/Normal/Heroic versions because the boss is always present and you don’t have that weird downtime. Clearly that fight was designed for mythic and then scaled down awkwardly for the other difficulties. And I know my guild is totally capable of defeating that boss. But we’re not willing to bench our friends in order to see it.