That's not a bad idea...
Personally, though - I already do that in the RPGs that I play (even Bethesda's games.) When I'm done with the character and ready for their story to be complete - that's when I go and finish the Main Quest.
My feeling about the end credits sequence is that, on a very personal level, I've felt that's what set Fallout 1 and 2 apart from other games. Even now, most games don't give you much of an epilogue at all. Back then, unless you were playing a Final Fantasy game, you basically got a "Congratulations!" screen and then maybe some credits rolled (and maybe it wasn't just a splash screen with scrolling text.) Even back then, I preferred games that gave a little bit of closure at the end of the game; it was just very rare for a game to put any thought into that aspect.
So the first time I beat Fallout 1, it blew my mind. Not only were they talking about things I'd done and people I'd helped (or hindered,) and how those actions affected things decades down the road, but many of the results were surprising. I thought I was helping one town, but my choice led to the destruction of two settlements, things like that. I couldn't wait to play again a different way just to see how things played out if I made different choices. The illusion was that the game was tracking every single thing I did. It made every choice, every dialog, feel very important - I wasn't just receiving an info dump when I talked with an NPC, I could potentially be altering the course of fate.
So a good closing sequence of some sort is... kind of near and dear to me. That's always been my favorite aspect of the Fallout series, and I really do kind of judge other games I play based on how it ends, in large part (to be fair I place a lot of importance in how a book or a movie or even a TV series wraps up as well.)
I don't think it'd necessarily have to be just a series of still pictures with narration over it, either. Something that just came to mind, but I'd be totally down for it would be that after you finish the Main Quest you could walk back to these settlements or talk to these NPCs and have a narrative kick in as you play. (For example, you go back to Arroyo after beating Fallout 1 and Ron Perlman comes on in the background telling you how Tandi becomes President of the NCR, etc.) Heck, you could even make it a Side Quest that unlocks after you complete the game that sends you back to important places to hear about what you've done (and why not - maybe some graphic wizardry to show you some of this more visually, a la a "flash forward" or something.)
There's just a lot of stuff that NPCs aren't going to be able to convey to you in the scope of the game no matter how long you keep playing the game. For example, if I beat Fallout 1 and could go back to Arroyo, Tandi's dad isn't going to tell me "thanks for saving my daughter. I bet some day she's going to be President of the NCR." He doesn't know what the NCR is - it hasn't happened yet, and he can't see into the future to know what'll become of his daughter.) And it would be kind of odd to save a town and get their well working only to have the residents tell you "thanks for helping us - too bad that because of this we're going to get wiped out by raider in 10 years..."
I'm all for playing after the Main Quest is complete. When the inevitable DLC comes out I won't want to have to load up a save from prior to completing the game just to play it - much nicer to just be able to load up my game and go there. So long as I get a nice epilogue at some point I'll be happy.