To me it's not so much a question of having decided and stuff. It's more a question of learning from experience. A max level player character that's gone through all the crap the New Vegas world puts you through should definitely be a jack of all trades to some extent as well as a master of something. Anything less implies that any Joe Average could've completed the quest, which has the unfortunate side effect of implying that the entire world is full of clueless idiots since they pretend to need your help. That's just senseless.
Obviously, it's a bit exaggerated to be a master of everything but it's not like there's a time limitation to the game, which means that the player character has all the time in the world to practice and hone his skills. Why wouldn't you know the basics of just about everything related to wilderness survival?
I'm quite sure you can build a character moderately capable in every skill, but that should of course result in the lack of higher proficiency in all those areas. And afterall, all characterbuilds should be viable to get through the game (barring some senslessly gimped jokecharacters).
I see the leveling up more as an abstract of mere human learning, which has its limits (and from skill perspective, a 100 means that noone in the world can be better - you are the absolute master of it). You can not learn to be a "master" of several areas in mere months (or how much ever the events of a runthrough are meant to last) and at the same time hone
all your other skills to the point of a general handyman. Sure, you can learn some basics in every, or a bit beyond in some, but being proficient requires specialization, which requires dedication to the matter at hand, which results in lack of training in other abilities. You choose what you specialize in, or if you create a generalist - and both should come at a cost. The reason of the levelcap and not being able to "jack" all the trades lies (imo) solely on gameplaybalance (which I regard as a higher importance than not needing to think about the realism of stopping to learn). Specialist and generalist both have their strengths and weaknesses over the other (or at least should have) - they support the different paths that the game claims to offer. Everyone is capable of something, but no-one is capable of everything. So by that, I see characters that have weaknesses (even severe ones) only adding to the game, rather than robbing the player of an experience. ...
got a bit carried away here, but nonetheless....
There are also certain questions involved: How much do you think is "capable enough" for this lvl-capped 'everythingman', and if this "capable enough" leads to almost all doors being open for everyone all the time, what is the point of having different skills in the first place?
E - I'm not really arguing against your points here, just giving my general view on the subject...