Without the theaters the studios would lose out on a lot of revenue because they are very much still trapped in the 20th Century. Studios are still very paranoid about piracy and reluctant to digitize movies for sale. They always require you buy a physical disc, or pay for it through a service like Netflix or Amazon where you merely rent them, not own them like you would a physical copy.
Sure, they can cut out the theaters and do direct to Blu-Ray or DVD but they aren't and if all the doom and gloom about physical media is true, they're even further behind in catching up with modern technology because physical media is supposedly a dead end.
So theaters still have a lot of importance to the studios's success but they aren't the sole source of income. Movies still trumpet their success from theaterical ticket sales, not from their Blu-Ray/DVD/streaming sales.
And the point is there are too many trailers that go on too long and take too long to get through. My first post I made pointed this out; 20 minutes before the movie itself came on, and this is after the movie was meant to start, not before.