I've seen the argument that unlocks complicate the game (they don't) and that they're unbalanced (kinda).
I agree with most of your points, but you can't just bullshit your way past this one with a few parenthetical statements stating your opinion with no evidence. Just taking one new weapon as an example, the Dead Ringer completely changed how opponents react to a Spy "dying", which, like it or not, is a "complication". Yes, a lot of the new weapons aren't drastically gameplay-changing, but I'm using this to refute your argument.
Even so, I do feel that the balance kept getting more and more out of whack with each new round of weapons (remember the original Backburner?), not necessarily on a team vs. team basis, but more so on the 1v1 level. I realize TF2 is a TEAM-based game (hell, it's in the title...), but we'd be lying if we didn't admit that a lot of matches end up being team deathmatch, and when a Scout with a bunch of shit (can't remember the specific items right now) is able to one-shot me, it gets annoying, regardless of how weakened he is for 30 seconds after killing me. Hell, the most recent update changing a whole bunch of weapons admitted to the horrible inbalance of some of them, so you're also at odds with the devs on this point....
And one final note...on hats. Yes, they don't affect the gameplay, even considering silhouettes, because those are still distinguishable from the bodies alone, but honestly...I just really fucking hate them. They look terrible, they take me out of the game (not that immersion is that important in a game like this, but it certainly is nice), and their primary purpose in the hands of most idiots seems to be to subject me to the most garish, unappealing visuals possible (this got a lot worse once painting items became a thing). I mean, kudos to Valve for making cosmetics in a FIRST-PERSON shooter a thing, but unfortunately that only results in trolls subjecting everyone but themselves to horrid color schemes.
I did see a mod to disable all cosmetics recently, though, so if I ever jump back in (it's been a few years) I'll definitely be trying that out.