Let's start be presuming there isn't a formalized Big2/Medium3/Little5 split. Because if that's the case, you're right that none of this matters and Big 12 (and BYU) are getting left behind.

So, barring that, which is more likely to make the Big 12 not a power conference - adding Memphis/Boise? Or not adding them?

Adding them certainly weakens the Big 12 a bit, probably drops the per team payout, and isn't great for perception. But it also protects against catastrophic collapse which (again ignoring the possible Big 2 split) is the only way the conference loses A5 designation, if some other conference poaches.

It's trading some guaranteed, but relatively small loss for guarantee against a possible huge loss - basically textbook insurance