I would agree in a situation in which I thought that the vanilla game was nearly perfect. Otherwise (and in the case of FO3), I'd say that you are pretty far off. Mods can change the gameplay. That's not just "extra."
Don't get me wrong - the modding capability of Bethesda games is certainly added value. But you're either going to buy the game because it's up to your personal standards or not - I've never bought a game I didn't like just because I was able to mod it into something I did like, for example. I consider that "extra," yes. Sure, it allows players to tailor the game to their individual tastes, but I'd still prefer it if the designers themselves had done so in the first place. (And I'm sure they feel the same way.) Obviously, you can't please everyone all the time. But that doesn't mean you shouldn't try to get it as close as possible, either.
I think that's a pretty weird perspective. If something about a game bothers you so much that you'd complain about it on a messageboard, shouldn't you be happy if someone points out that you can make the problem go away? Shouldn't complaining be a means rather than an end?
I, myself, am not on here simply to hiss and moan about problems I find in the game, however. I'm perfectly aware of the existence of mods out there - if any exist that fit what I want out of the game, then I've already downloaded them and am currently playing with them.
Everyone's here for a different reason. Myself, I prefer to discuss the various ins and outs of the game's design - any criticisms I level at the game I try to suggest solutions that everyone would be happy with, something that could be (ideally) unanimously approved of. If there's a mod that actually does that in a particular case, then that serves as a good example; but (with all due respect to the modders out there,) those are few and far between.
For example: I'm not a huge fan of the level cap in the game. I like gaining levels. But the real problem goes much deeper than how often and quickly you gain experience points, or the thresholds at which those net you more levels. I can (and have) download a slower levelling mod, but to really "fix" the real problem means pretty much reworking how the entire ruleset is laid out. Which could concievably be done with a mod, but that itself would affect other things that would then need rebalancing, and so on. Sort of a domino effect, there.
When it comes to mods, I'm here to discuss the various qualities of the game itself - what went wrong, what could be done better (and possibly ways to attain that goal.) If a mod actually does solve the entire "problem," then so much the better. But otherwise I don't find it pertinent to the discussion.
In other words, if I were to say "Hey, I wished in Fallout 3 that I had a spraycan so I could keep track of where I'd been when exploring an abandoned factory or cave," and someone came along and said "Well, there's that spraycan mod you can download," then that's great. That actually does have relevance. I could then point at that mod and say "Well that works just great, I hope they put something like that into Fallout 4 so I won't have to wait around for a mod that does it." But if I say something like "The level cap problem is caused by deeper underlying issues with the balance of the ruleset itself - the skill cap, points-per-level, etc," then saying that there's a mod that slows down the levelling doesn't really "fix" anything, and isn't terribly pertinent. It's only putting a bandaid on a wound that's internally bleeding.
Not to mention that if I do have an issue with the game that can be solved by downloading a mod, it's still (to me) not as ideal as the company making a game that suits my needs already. Sure, that's unrealistic and unfair. But I'm here to share my own opinions; and I'm certainly not going to lie and say that I wouldn't like it if they just tailored the next game to exactly suit what I wanted out of a Fallout game. It's up to Bethesda to look through these threads (to the extent that they do, at least,) and decide what's in their best interests to listen to for their next game. But it's certainly not going to have anything to do with my wants if I don't speak up and ask for them, either. Which has nothing to do with the availability of mods.
The "perfect" game doesn't have need of mods - it's already perfect. Sure, that'll never happen. But ostensibly that's what every game designer is striving for, nonetheless.