The problem many on this thread are overlooking, is this:
From back in the cheap seats, waiting for the captain's EV announcement means having faith that:
- The folks up front in the cockpit are not incapacitated and are still in their seats,
- The folks up front can clearly see what's going on, not only outside the aircraft but in the cabin as well.
- The PA system is still working correctly.
That's a fair measure of faith to ask for. DIY evac might be the wrong choice in some cases, even calamitously wrong, but it is not an entirely insane one.