What are you proposing in terms of difficulty? The only difficulty in this game is in memorizing the mechanics and rotation. It's literally just memorization. If you're saying that memorization isn't difficult, then there's no difficulty in any MMO that I'm aware of. Half the time, when I see someone whining for more difficulty on MMO forums, they're complaining about other players getting to have fun and complete content without having to memorize all the extra crap that they did. The other half of the time is players complaining that eight button rotations just aren't challenging enough for them and that SE needs to throw another eight into the mix to make their job interesting again.
Sorry, but neither of these playstyles appeal to me. I memorized tons of useless garbage on the way to getting my college degrees, and I memorize tons of stuff for work already. At the end of the day, I don't want to memorize a bunch of meaningless garbage for a video game. I'm fine just learning a handful of mechanics and responding as they come on the fights. When I was younger I felt differently, but now I don't feel compelled to study all the fights as if my entire future were riding on them anymore. Some people like to do that, and that's fine by me. What's not fine is when they turn around and complain that because they studied the fun out of the game the developers should make the studies into baseline requirements for everyone so that they can feel challenged again. Like you just said, it's not difficult; it's just tedious, annoying, and unfun.
Same with the 16-button rotations. At what point is enough enough? My favorite aspect of Classic WoW was the simple one-button frost mage rotation. That freed me up to exploit the heck out of marginalized abilities like Blink and Polymorph. I'm not saying that we need to go back to one-button rotations, but I'm honestly fine with four-button single-target rotations and four-button AoE rotations. I don't find joy in juggling half a dozen ogcd weaves and timers.
If you want a difficult game, feel free to ramp up the difficulty for yourself. Take all the consolidated skills, and bind them to multiple buttons so that you can make your six button rotation into a nine button rotation again. Stick to Extremes and Savages that require you to maintain your focus through every fight and optimize every aspect of your gameplay. Just let the rest of us enjoy the chill game that we know and love. The game's not being dumbed down. You're just older and more experienced than you used to be.