Yes mafia absolutely can sit back and watch town blow itself up, it happens and it wins for scum, but when town is actually on the ball it is much better play to actually be in the thread, planting ideas, planting wagons, etc. than sitting around hoping townies (who have adjusted themselves to scumread exactly that kind if behavior) don’t notice you. And if you do have a bad situation like that in a town, you can identify what’s happening and what slots seem to be coasting and recover if town gets itself back together.
Unfortunately town does just blow itself apart with coasty scum because they just aren’t playing well often, but scum will do much better much more consistently if they get in the fray and incite even more destruction, and it’s ironically also much harder to identify because of the type of things good townies are looking for and because you naturally blend in when you’re blowing town up while a majority of town is also blowing town up.