Beth games, being sandbox EXPLORE games, have a lot more content then whatever CRUTCH storyline they may have. The main quest in FO3 is for people who ccan't/don't do sandbox games. If you guys spendt 30 hours on it, you missed 80% of the available content.
I don't think that Sandbox games by popular definition even exist; in every game you're restricted by the gameplay mechanics, and by the developer's writing. As previous posters pointed out your choices in the main quest and side quests are actually quite limiting; there are no infinite possibilities in Fallout 3 the only difference is that you have the option of exploring in greater detail than the average RPG, and that doesn't make Fallout 3 a "sandbox" game. Quests in Fallout 3 are actually more linear than quests in Fallout 1/2; in those games you usually had many ways of dealing with a situation while in Fallout 3 you usually only have two (a good, and an evil path), and sometimes you only have one. Being able to "LARP" in Fallout 3 doesn't make it a sandbox game either as you can do that in any video game if you really want to, and it has no impact on the game at all.
You operate on some sort of opinion that storylines hamper RPGs; you have a right to your opinion and I respect that since you present your argument better than some folks, but I'm afraid I don't agree with your stance at all. Storylines generally add to the experience, not detract from it... they're fluff, window dressing; they make the game nicer, they're not a "crutch" as you put it. If you take away the story from the game then what you have left is a soulless shell; you may believe that LARPing is great, but no matter how much you talk to yourself it doesn't have any impact on the game what-so-ever because you are bound by what the developers present you. I didn't like Fallout 3's story since I think it was poorly executed, and the religious undertones didn't really click into the setting, but I'll be one of the first to say that the final product would have been a heck of a lot worse without the story.
I fully support using your imagination, but personally I think it's better to put it to use in creative writing or visual art. I don't see the appeal of LARPing, but that's probably because I prefer to write to create my own stories. When I read books, watch movies, or play video games I prefer to see someone else's story, not substitute my own.
As I said before, just about the only way I won't buy FO4 is if it IS NOT a Beth style sandbox/explore game.
Didn't you play Fallout 1/2? Are you saying that if Fallout 4 were in the same style as Fallout 1/2 you wouldn't play them?