>In heads up no-rake NLHE where GTO exists: do you agree that anyone playing GTO has nonnegative EV against any player regardless of their strategy?
In heads-up, no-rake, NLHE implicit collusion is not possible, so yes playing GTO has a non-negative EV.
>I agree with you that you can probably construct some pathological multiplayer situations where collusion forces negative EV on a "GTO" player - assuming GTO exists in this setting.
GTO always exists, but in the presence of implicit collusion it may result in a negative EV.
> feel pretty strongly that this is not representative of practical reality.
GTO is not representative of practical reality. Very very few people, if anyone, really plays GTO poker consistently. It's an excellent theoretical result, and using GTO solvers and simulators can build up good intuition, but the idea that there are these poker savants playing GTO poker in 6+ handed games is mostly a myth.
Also your claim that you have never seen anyone play against poor players have a negative EV kind of gives away that you are likely overconfident and possibly naive about your position.
I highly doubt you have been in a position to calculate someone's EV while playing poker, or assess whether someone is playing GTO poker against multiple opponents who are bad. And assuming you did possess this amazing power, GTO poker does not guarantee against losses against bad players over any particular night or week of poker. So if you've never observed that then you are likely biased in your observation. You forgot/underweighted the scenarios where someone playing really well lost money to bad players and overweighed the scenarios where someone playing well made money against bad players.
My experience is that talk about GTO poker is mostly used by people to add a more sophisticated and technical dimension to poker, people like to say that a poker play was GTO because it makes it sound authoritative and you can geek out over it, so to speak. People do the same thing across many hobbies including sports.
In reality the best advice for playing poker if you want to be profitable is to find fish and play against them. The sad truth is that there's no shortage of gambling addicts to take advantage of, or people who overestimate their abilities but lack discipline, or heck are simply rich and play poker socially and have no problem losing money and have fun doing so.
That's far more likely to yield profitable results than pretending that you can play an incredibly volatile and dynamic game like poker with anything remotely resembling game theory optimal.