Garrisons will disband just like any other unit if it goes long enough without supply.
Forts are good for war index points. Switch your country over to being judged as peaceful then war index makes no difference.
What does 150 forts do for you? Brings your war index up and IF you have them garrisoned, they prevent someone from painting the map without destroying the garrison.
If you don't have them garrisoned, then that's just 150 places for someone to strip your defense helos. A weak garrison is just as bad as no garrison.
So, you've created 150 additional targets you have to defend just to keep someone from painting the map.
Do the numbers, protect your population centers, state corporations and military bases (including space centers). If they can't destroy those while the numbers are going to get low enough to scare you .. they can't win the war and you can always take an occupatio
I don't think everyone keeps all there garrisons up though. From scoping out FB it looks like the heavy weights defend against sneak attacks with a much smaller number of divisions. Then Probably just throw up the garrisons if a war starts.
You don't HAVE to have garrisons to defend, you can do it with land units positioned correctly.
However it's not so much triggering a response as it is triggering a maximum response.
On one of my targets you're dealing with the garrison, one land unit and 3 fed Air defense units. Drop the garrison and it's just one less thing responding to an attack.