Obviously I'm not
@el-remmen but I have increasingly become leery of licenced settings myself, for a couple of reasons beyond the ones you note (i.e. never
really the protagonist, specific canon events).
1) Players often have completely encyclopedic or just really in-depth knowledge of these kind of settings or these settings specifically. They're not usually jerks about it - they don't say "Uh you can't do that!!" or "WRONG!!!" or whatever, but they do love to bring up and discuss lore, often irrelevant-but-funny lore (SW and Marvel/DC are the absolute worst for this), or get overexcited by lore characters/events or the like. I do find that the further out from the "mainstream" you get usually the less this is a problem, but still can be one.
2) On the flipside, if you are a player, even if you never mention lore, often you can see it being mangled in real time, and whilst that's survivable, it can be a little frustrating to realize the DM doesn't understand how X device works in lore (even though it might be very well-established), but do you really want to disrupt things by arguing with that? Probably not. And it can sort of chip away at the fun of a game, or act as an anti-immersive distraction from it.
None of this is unfixable, but it is enough that I would never want to run a Marvel or DC-set supers campaign as a result.
One of the few places I think this would mostly work out as a net positive is probably Star Trek, but even then...
This sounds like a general complaint against a certain class of anti-social player and not against the concept of a licensed setting itself. The complaint sounds like "licensed settings bring out common anti-social behaviors in certain classes of nerds". And I feel like that's something that you can head off at the pass by knowing it is coming, or which would only be a problem with some groups.
Although you make me not want to be a player in a Tolkien setting just because I realize how hard it would be for me to respect the poor GM if he were "wrong".
Part of the reason I asked is that for the last 3 years plus I've been running a game in the Star Wars setting and none of the problems thus discussed are relevant. Yes, I do have 5 nerds I'm playing with who all geek out about Star Wars, but fortunately none of them are obsessive EU geeks and I seem to have the most lore/setting knowledge of the bunch in part because I'm the one doing all the research to prepare the games. Also, I'm running a game in a period that until recently was poorly explored - 3 to 4 years after the events of "Revenge of the Sith" and in a part of the galaxy which has not been explored heavily in the live action media - the Aparo sector (and now a bit of the Wyl sector).
Also when I started the game I very specifically called out a few lore related issues because I anticipated possible problems. First, I divided all Star Wars lore into three types:
a) Canonical within the campaign. These events either have actually happened, or barring PC intervention will happen, but in any event accurately portray how things in the game universe work. Note that time scale compression is assumed to be a thing even in telling of canonical events. For example, in the game universe the Death Stars attack on Yavin occurs two weeks after Luke, Han and Leia escape the Death Star because it takes that long to move the Death Star from Alderaan to the Gordian Reach even in hyperspace, something that is not obvious to a typical movie goer but which on the other hand doesn't contradict what is depicted in the movie.
b) Partially canonical in that the events may have occurred as related in broad outline, but the actual details may have no more relationship to real in game events than a typical Hollywood movie. The Prequel Trilogy and the Clone Wars cartoon is of this standing in the game. "Based on true events." But, for example to illustrate this, I explained before the game started that in the game universe Anakin met Padme when both were 15, and that the Clone Wars lasted 10 years and not 3 years. Thus, Anakin is 35 at the time of "Revenge of the Sith" in my timeline, and will be 57 at the time of "Return of the Jedi" in my timeline.
c) Not canonical at all. These events either never happened or never will happen, and do not accurately reflect how things work in the game universe.
Likewise I told the players that the future isn't fixed, and that hypothetically they could disrupt or change the canonical stories, but that it was a big universe and the most likely thing that would happen is that they'd become big fish in their small corner of it. As things stand, they've been only lightly in touch with the established lore of the setting. They've met several canonical characters, but not any that a casual fan would be familiar with. They've influenced canonical events in the EU, but not any casual fans would be familiar with. And it's interesting that almost everything I've declared as fully canonical is in the future of the setting. It's less about how it informs the current story than it is setting a standard in tone.
Fortunately, I've had no players object to this, as none were huge fans of the particular stories I have openly deprecated. I do have one player who is a big fan of "Crimson Dawn" who make be disappointed to discover that in my timeline, Darth Maul is truly dead (his brother is pretending to be Maul for the cachet of such claim), but that's not come up or maybe he'll think it's a cool twist if it does. But on the whole I don't have and haven't had any of the problems you are describing.