Very valid concern...and agreed it would be a good problem to have.
The franchises that see sustained success (i.e. they're almost always in contention to make a solid playoff run and have a chance @ S
build and maintain stability by having good HC & QB. They have good scouting & management, as well, but it doesn't seem to necessitate the same consistency as at HC & QB. Obviously better to have elite @ both, but one can compensate for the other a bit. Andy Reid sort of comes to mind...he and McNabb in Philly, 5 NFC championship games. Then in KC, he probably pulled Alex Smith's performance up, while Mahomes is now probably pulling him up a bit.
That said, when you have those 2, other parts come & go and the team can generally remain competitive, even during retooling. The highs and lows are both higher than when you don't have those things.
Problem is, you don't know whether you've built that stable combo until it's played out a bit. Luckily (in a sadly awful way), it doesn't seem to take way too long to understand when you haven't.