Personally... I believe that its best to abruptly end it while one still wants to play, rather than to allow the player's last memories of the game to be that they got bored with it. Limitless free play [of this kind] devalues every aspect of a game IMO; and seems like it would be a nightmarish minefield to the creative writer, and their endeavor to craft a compelling story.
i can see your point, but bethesda games are fun to play for years and they don't have endings, its not the game having an end that makes it good or not good, its the entire game experience, its not ending slides or an abrupt ending....listen to todd howard sometime on his many interviews on how bethesda approaches their games and their philosophy on game making, its about the entire experience he says, from before you even buy the game, to all the way thru it..so its a philosophy some game developers have to "end" their games, but lots of really good games don't have the game end after main quest, and lots of lousy games have game endings, but i think it just depends on how good the game is overall, its not the ending that makes it good but its fine to have it end if its aporopriate for the story but to just make all games have those endings as a philosophy i don't think makes any sense, simply because the best games i have ever played so far over the years are games that don't put a lot of restrictions on you.