What are you even talking about? You already have to evacuate a garrison upon seeing an anti-garrison capable unit approach. The difference is that now you can get away completely undamaged even against flamers. That isn't fair. That isn't tactical.
You have to evacuate if there is a flamer, that is true but you have then forfeited the house and all its advantages to the enemy and thus you are now in a weaker position. You seem to ignoring that fact because you are obsessed with needing to do damage to the garrisoned squad, which by the way you want to achieve by simply walking into range of the building..
As to grenades, you do not need to exit currently until you see the animation, which is different to your suggestion because it involves counterplay and does not automatically guarantee damage.
It has all upsides and no downsides currently, as flamers and grenades NEVER hit ANYONE in a garrison, just literally never unless you're at like below 2000 rank or something.
Again, the objective is to take the house and gain the benefits off the garrison for yourself.
That makes garrisons have only upsides and no downsides. That isn't proper design. If you garrison a squad, there needs to be risk involved.
Houses are a cover mechanic, the downside is if you are outplayed you loose that cover to the opposition.
Also how is this different from building a trench or sandbags. Should unit behind sandbags be imobile so your grenades always hit...
Also, if you'll notice, I mentioned how models exit garrisons one at a time in DoWII. That means you aren't going to get wiped, but you WILL take some damage from grenades and flamers if the unit gets in range before you start leaving the garrison.
Again, your obsessed with doing damage and denying your opponent any counterplay. You also continue to ignore the effect high damage nades/satchels/bundle nades will have if you are guaranteed a hit.
I don't understand how you can think that's unreasonable. Staying in a garrison until literally the last second is utter trash game design, completely non-tactical.
I'm not sure what your definition of tactical is TBH. Staying in a house, or behind cover or even remaining in the fight to get a faust off when your low health is a tactical decision which also has risk.
Just because you decide to attack a garrison with the correct counter does not mean you are automatically entitled to a reward. Your opponent is allowed to respond. If he stays and fights he will take damage and he may or maynot win, if he leaves the house he gives up his advantage. It really is that simple....