If tennis is so easy, then it wouldn't make sense for me to even stay at this forum, since I will be already playing futures and tourneys and get my name out there on the screen. I agree tennis is not easy.
However in this forum, I don't think putting tennis into categories and broad buckets where everything is sweeping into a very wide generalization is really helping anyone here.
Your own words: "Now play someone 3 levels above you: how come the same outcome doesn't occur? Because he's way better than you: he can hit shots that you can't retrieve; he can disguise his shot; he apparently never misses; he puts you in uncomfortable positions. You're busting your butt just to keep in the point and he's not putting out more than 20% effort and can end the point anytime he feels like it."
You are putting words into my mouth which I never mean to say at all, and I think you didn't really read my comments since you are writing more reactionary on certain keywords I think.
I recognized that people at higher level has done more work and effort to get to where they are, and low level tennis players are low level for a reason. However, normally high level players will not play low level players, if you are really put into the spot where you are hitting with someone really good, your objective is not to win, since that is not something you practiced against. Your objective is to try to change up the game, adapt to their game and try to find an opening that you can manage and learn. The high level opponent already know that they will win no matter what they do, the good reason they are still playing, is probably want to see if their opponent put into the the effort to try to get the ball in, play smarter. If you only try to challenge their strength which you are weak against, they will only laugh at you and proceed to steamroll you. Instead of doing that and gain nothing, I think a better way to think about it is to have fun and improve at the same time to make the most out of this hitting session.
Let's give an example, you mentioned the 20% is normal rally shots and the opponent has very very fast forehand and spin that you have no time to react and pick up, instead of trying to "hit harder" which you know you will make more errors, maybe the best way to adjust is to stand further back from the baseline, and not trying to hit on the rise? And if you are doing that, might as well try to return with more high percentage shots with higher bouncing "pusher" like shots with added spin to slow down the pace without compromising your positioning and perhaps slowly approaching the net and try to find a easy shot to hit a ok volley back to the middle of the court. Perhaps play up the variety and start slicing to the side if you can and make them move even though you know they will always get to the ball and maybe hit a winner but at least you tried.
Anyway, good that you share your opinion of it, perhaps I should make my comment more clear in the future.