I don't know what the solution could be, but recruiting 20+ people for core and bench is starting to look like an impossible task at this very moment. I would even speculate it's easier to recruit 40 people in classic than it is to recruit 20 people for mythic in retail at the moment, and that's quite ridiculous.
My solution proposal would be obvious, reduce the number of people required for mythic content. 15? 10? again, this is the only thing I can really think of, as making mythic content scale-like LFR, normal, and heroic does would just be a balancing nightmare.
We are on a "full" pop server, US Thrall with plenty of people. The problem is related to not enough talent being out there to fill the roles. Is the content too difficult for the average player? is Raider.IO part of the problem? again, I don't know what the solution could possibly be, but there's definitely something going on with recruitment that is making it really hard to make a consistent roster.