Everything you say is irrelevant--garrisoned walls already don't decay/get captured. Walls only get captured when they are entirely in enemy territory and unmanned. Men can't even exercise a capture mechanic on walls.
If you think unmanned walls in enemy territory shouldn't be captured then you should tell all the Turks, Greeks, Arabs, Slavs, Brits, etc. that they don't actually have control of the old Roman walls in their countries because the Romans (who no longer exist) still control them.