Operators are a problem. Obviously, they remove players from the central power fantasy of Warframe. Why are you playing WF? Probably because you really enjoy being a zippy space ninja with guns and superpowers flying around like a 'roided out acrobat. Being forced to exit that and go kid-mode where you're slow and only do maybe two things is excruciating. They can make Focus and Operators as OP as they like when you've ground out most of the trees, but that won't change the fact that they're boring and at odds with the core gameplay flow of WF. Also just a tad ironic that you're basing an argument on gameplay diversity when Operators are literally only used for energy-botting outside of missions where you're required to use their dash or beam to do a thing.
Warframe ran for years without the basic gameplay becoming boring, and a big part of that is because DE put in a lot of effort to make sure that weapons and 'frames were as distinct as possible with as many viable options as possible. Frankly, they did a good job-despite most 'frames fitting into a handful of ability "roles", they look unique and handle in very distinctly individual ways. Weapons are slightly less well-off, but only really because some early weapons (looking at you, burst rifles) have been left by the wayside. You can have 60+ weapons forma'd out that you use regularly and barely any of them will closely overlap with the "feel" of each other.
There's also the lore problem. Regardless of whether this has actually been planned or not, Operators were a massive change-up in how we view our characters, and not a good one. WF, like most open RPGs, allowed a very large range of diversity in terms of how players viewed their characters. We knew something was going on because of all the hints of an in-game equivalent to the player as a controlling factor and discrete identity, but it was largely left up to the player to interpret that lore and envision their character.
TSD told players "whatever you imagined was wrong, you are all now whiny immortal teenagers who are massively hypocritical, also you now have to play chunks of the game as immobile insta-dead fleshbags instead of using your Warframes". Yes, you can turn off the godawful voice lines. That doesn't mean that TSD and TWW, however good they might have been otherwise for lore and gameplay, didn't completely trash the notion of player control over characters. That, even more than the crappy mandatory Operator mode, is why Operators and Focus are so widely hated.
It's a shitty grind, players are forced to stop doing the stuff they enjoy and use a wonky system that's equal parts dull and OP (seriously, the energy regen is basically mandatory now) if they want to complete content, all player agency is stripped from the creative construction of characters (oh boy, we get to choose which shitty hairstyle and latex suit our space-kid gets to brood in!), and all of this crap is taking time that could have instead been used to rework underperforming weapons and 'frames, build new content that you can actually play with your WF and progress in the larger game with, &c.
It's not that there is "no" progress in PoE. It's that all of the progress is tied to a shitty mechanic that a big chunk of the playerbase despises, in no small part because it feels like DE is trying to cram it down our throats while ignoring other things that need to be done. They can't even walk it back, it's been too long since TSD and too much content is based off of the Operators. I just hope that The Sacrifice is going to be giving up space-kid in exchange for an Umbra.