I actually liked FO3's original ending, it was the Broken Steel retcon I found stupid; it didn't even make sense.
And what do you mean 'pay to play'? They could always do like in FO:NV and put a warning at the 'point of no return' although personally I think that kinda ruin the surprise.
In Daggerfall and Arena you could not play after you finished the MQ, infact in Daggerfall (although it has many endings, all of them happen at once and are considerd canon as far as I know);
Spoiler You die, the Numidium goes on a rampage and the protagonist(you) gets trampeld to death.
This. Okay, I just gave my life so that the people around washington can live better lives with fresh unradiated water. I finished my parents work. I'm a hero, who gave his life for the greater good. I left the safty of the vault to find my father, but I found a world no one should have to live in. However, durring my time on top, I was able to save so many people. I was able to see the night sky with my own eyes, not in a book. I'm okay with dying, because it will help not only the people now, but those to come, and in time people may eventually come out of their vaults, and the world will return to the way it was. The experiment was a success. I may have just saved the planet and all life on it. Man, I'm really a badass kinda guy!
Oh wait, I'm magically alive now with no real explanation besides luck just because? Well...that kinda...er...takes away from...ya know...the hero feel I was feeling. I mean, I'm still a hero...but, well...I just got nailed with an ungody amount of radiation....how am I still here again? Why are my outsides not looking like my insides and why do my insides not look like nuked mac and cheeze?
I love playing after the main quest just like everyone else. But if I throw myself into the fire to save everyone's lives, I should burn. The hero giving up their life to save the world has been done to death, but jumping into the fire to save everyone and then being pulled out with no negative effects at all is even worce.