The first filter on catcher defense is pop time to 2B. If you can't get your pop time down to a reasonable level, then every runner who gets on base makes it to 3B. This is one reason why position-switching in WAR as a basis for catcher replacement level doesn't work. People who can play C can often defend elsewhere but the reverse is not true. If you move your 1B to catcher, then every leadoff walk is a leadoff triple two pitches later.
The second filter on catcher defense might, ironically, be how well they hit. If a player can really hit the ball, he will be moved off catcher to a less demanding defensive position so the team can have the bat in the lineup every day.
The third filter is handling the pitcher - game calling, blocking, framing. You can offload pitch calling to the coaching staff if needed but not blocking pitches and keeping the runners from advancing. If we see automated balls and strikes you will see catchers change their stance to help with blocking and pop time. They won't care about framing low pitches anymore.