slow games are made even better when you can only shoot while aiming (REvil, project zomboid and metal gear solid do this)

Not sure about the later ones but you can shoot without aiming down sight in mgs1 2 and 3. In fact in mgs1 whether or not you can ADS at all depends on which weapon you're using. These games don't conform to the regular concept of ADS, instead their "ADS" is simply a combination of raising a weapon while being in first person (each action is its own toggle), so not really the same thing. You're right that some weapons automatically bring you into first person and therefore functionally force you to ADS while using them, but most don't work like this.