No, I mean the campaign AI is a terrible judge of party strengths in live battle, so the player will be able to drain garrisons by simply engaging near castles, slaughtering them for minimal losses. Meanwhile, the campaign AI is also like a shark with weak garrisons being blood in the water, so this strategy can easily allow them tofollow up your garrison-draining activities with an actual siege.
I did a kinda-sorta version of this by running around with 200 archers, besieging towns and castles, then reaving the defenders on the walls with all my arrows. Obviously there was a cost to that, but in a open field battle I could do it even more quickly and easily, almost certainly with fewer losses. Making it even easy to seduce the garrisons out would seriously distort campaign balance.
The campaign AI has issues that go beyond this, you could also siege a settlement on the other side of the territory and the AI would gather and send armies your way while armies gathered by your faction go around taking other settlements. When they come in to defend they also track you longer, so you could even kite them away from their territory, maybe getting them to starve.
There is a YouTuber called Spiffing Brit and he posts a ****load of videos exploiting games, some get patched, other are just not worth the trouble. In my opinion TW shouldn't be constricted into implementing things only if they feel they can make it exploit-proof, there is only so much they can do, and people tend to find a way to cheat if they really want, either by using the built in cheat mode or downloading a mod for that.
Maybe instead of sallying out for field battles, garrisons could be constricted to supporting armies that attack besiegers, and even this could be exploited, if you won the field battle with minimal casualties it would make the siege easier, but I think this is better than watching a 700 strong army sitting outside while a 800 strong army takes their settlement, even though there are about another 200 troops right next to them inside the settlement.
Not related to the argument, but, did you have fun running around with 200 archers? Would you put a cap on troop type so it couldn't be exploited? I would add this to the Swadian Knights "issue" we had in Warband. Should TW had made it so mixed armies in terms of types and quality could've stopped a full army of heavy armored knights? Wouldn't it have made the knights useless in AI armies that can only field a few? If a full force can be stopped by lesser troops, what chance would only a few have of being useful?
My point is, even if TW decided to nerf archers, the nerf would hit the AI the hardest, as soon the players realized which unit is overpowered now, we would field full armies of that unit.