because it's simply that rare

It's rare because people either don't care about getting there or because people make it sound entirely out of reach, when it really isn't. Sure, there's a limit of just how many people can stay in T20, but it's certainly realistic as a goal.

spending exorbitant amounts of feathers that would usually be better used elsewhere to expand your available good units.

It's entirely possible to earn more than 100,000 feathers in a month without summoning at all. Maybe even 200,000+ if you have time/willingness to HM farm now.

You don't need a full team of +10s to reach, let alone stay T20, so let's say 4 months to get the merges you need. That's not unattainable. Maybe 6 months if you don't 100% focus on them. Still reasonable, especially if you made it a goal months ago.