First of all factories cost twice as much as razor kisses. Razor kisses have 25% less hp but still WAY better hp/cost. I see no reason to make factories as tanks then as you can repair them alot slower and they cost more.
Now having a durable closed factory wont change much about the gameplay. If you are getting rushed you already lose all your eco which is a big disadvantage already. Its really unforgiving to lose all your units at the first minutes of the game especialy when its your first game.
Having a durable factory leaves some chance to nubs when they get rushed so they dont lose everything they have. Most of the time they leave when their factory get killed then later I actualy see their team winning the fight.
Then again what unbalance does a durable factory create?