Replying to you personally, but the others in general. I agree that we haven’t seen (either with Utd OR England) a system that is set up to play his strengths. Whereas you could argue the two players I listed as potentially becoming elite; Amad and Garnacho, probably are helped by Utd’s style, especially with a significant number of counter attacks. I may just be plain wrong, it will be very interesting to see why he’s like (potentially post- ten Hag) if you end up playing a more Guardiola type system. We shall see. And yeah, fair point about Pogba, at one point he looked like he could go on and become a once in a generation player, but a mixture of bad luck and bad attitude scuppered that.
In assessment, it's not sacrilegious to look Mainoo's way expectant of seeing a player you can determine is as special as the clamour around him warrants.
Depending on what you like or deem important in each type of midfielder, there'll be aspects to his game that impress or underwhelm, perhaps a smattering of both. What most would conclude is that whatever he is isn't the finished article; his final position on the pitch hasn't even been consensus yet with some believing he should be a #10, others an eventual #8, and others still, a #6 who is the first receiver and ball progessor.
The truth is, he isn't particularly definable right now and exhibits traits across all 3 major positions in midfield as well as reasons why he currently lacks in each position:
#6 - he is not yet comfortable playing expansive passes over distances beyond 20 yards or so. He did this effortlessly in the unders but has not yet shown any desire to do so extensively in the PL. Also at #6, whilst being the first receiver is great for the team - as it assures solid and dependable ball progression - it can be perceived as a waste of Mainoo's connective ability and inherent ease with ball carriage straight through midfield.
Mainoo has some De Jong traits, but they are fundamentally different players, and whilst De Jong is in his element affecting the game from deep, Mainoo currently does not have anything like the stamina or certainty of purpose (De Jong knows exactly what he's going to do with the ball before it even gets to him, and driving into and through congested midfields is something he does with no effort or exertion) to play that role optimally. De Jong can make bursting runs repeatedly and throughout 90mins, Mainoo would be good for maybe 4, tops and would be out of gas if he had to go on a repeat run before recovering from the first.
#8. Massive issues with stamina. #8 is the most active and perpetual and demands the most stamina. Mainoo cannot complete an all-action #8 performance and there is almost guaranteed to be in a distinct decline in performance from 1st to 2nd half. His current window is 60-70 minutes, and after that, he'll be a shadow of himself.
#10. Movement and certainty of purpose. More so than even an #8 or #6, the #10's worth anything have their head on swivel and are switched on to their surroundings, knowing exactly what they will or can do proactively or reactively as a situation demands. This is the sharpest mind on the pitch, and it should show. Every amble or drift is supposed to be packed with intent to suddenly explode into action, be that a sudden pass between the lines, an explosive run, even over a tiny distance (simply to create that window of opportunity to pass, shoot or open up space) and Mainoo, whilst having the ability, doesn’t think or act like a #10 and is currently far too passive and retentive (perhaps reticent) to be one.
So in each position, he can be said to be lacking in key aspects that pure midfielders, who we can say are and will always be one thing first and foremost, do not.
Others have mentioned some of Mainoo's spiking attributes, but his biggest ability by far, imo, is his combination: passing, dribbling and movement in tandems, trios or even 4-man chains. This is where Mainoo is already one of the best players active and where his wheelhouse is. Put him in a team with like-minded and equally skilled combination players and he is unplayable until his stamina depletes. This is the Mainoo that there's a fear of losing down the road as clubs that actively seek that kind of player circle and will tempt him with offers throughout his career.
Put Mainoo in a team that plays combination football and a lot of his flaws are masked and what he excels at becomes undeniable. The few times England managed any chain of worth, Mainoo was a key component and it will be the exact same at Manchester United.
The biggest failing of the club this summer has been in not facilitating his game and continuing to have him a plough a lonely furrow that has him rely on base fundamentals that revolve around athleticism that don't just magically appear - his stamina should improve over a period of time, but not in a small window where he had barely any downtime.
Pogba isn't at all the same kind of player. What he is was always clear: an outrageously skilled individual who actually struggled to play consummately in a team. Mainoo and Pogba would actually be better paired, one bringing razzmatazz, and the other quietly connecting the team and being a constant thorn for the opposition.