Only if you don’t care about getting CE, HoF, or any other achievement related to killing it with the static 20 player group.
From having engaged in this kind of topic in the past, current mythic raiders have been pretty unanimous in suggesting the achievement (and related benefits) are the only reason they’re raiding mythic. Unless those previous discussions have been inaccurate, my proposal doesn’t change the incentive for current mythic raiding guilds to run in the locked 20-player mode. This added flexibility is simply a way for groups that don’t have exactly 20 yet get AOTC early in the season to not have their season end at that point, unless that’s what they want. And as a side effect, there would likely be players who choose to pursue the standard mythic experience after getting a taste of it with their current guildmates.
The issue here is with the assumption that everyone who isn’t raiding mythic are incapable of doing well in mythic. There are a lot of barriers to raiding mythic today, some which have nothing to do with how well you can press your buttons and do mechanics. I’m just suggesting that lessening those barriers for a version of mythic that intentionally excludes a significant motivation current mythic raid teams have for being there today would be better for much of the community.