This just further proves to me how underrated Maguire is.
Reads the game better and positioning? Maguire prevents a lot of danger with both his reading of the game, and his positioning, basically every game.
Uses his body more effectively? How is that even possible when Martínez is so much smaller?
Martínez is simply just an inferior profile as a centre-back, mostly because of his size and height, but not solely because of these things.
He is proactive, he is tenacious, he has good timing to help win aerial duels, he's good at improvising, etc. reads the game well too, I'm not denying it, but if you compare the two profiles, a 1,75m tall, not particularly fast CB will never ever be a more valuable profile than a Maguire, de Ligt, Dias, Botman type, who are:
-tall, tower over most opposition players, aerially dominant
-very dominant physically
-good communicators and organizers
-great box defenders, basically box dominators
these are all things Martínez is simply not excellent at or is incapable of being excellent at due to his physical limitations. And all the things you listed for Martínez are absolutely true for Maguire as well.
Martínez will never be able to absolutely dominate a game like Maguire did against Fulham a few weeks ago, for example, or how Maguire regularly dominated opposition when he was our first choice CB under Ole.
Martínez is very valuable when you have the ball, because his press-resistance as a defender is second to none, and he can probably outpass any defender in world football as well, bar probably Trent, and maybe Stones/White are worthy rivals of Lisandro.
However, just looking at it purely defensively, he has a lot of shortcomings, and is simply not built to be a centre-back. The next manager will probably use him at LB and that will be the end of him being a mainstay CB at United.
And please don't even mention the high-line nonsense. Maguire has proven himself in a high-line time and time again. It's even funnier that when Maguire was basically the first name on the team sheet in Ole's two full seasons, we always had one of the highest defensive lines in the league. When ETH came in and benched him for Varane, we had one of the lowest lines in the entire league in both of ETH's full seasons so far. He might not have a lot of pace but that's not the only thing you need for a high-line, you just need to put your slowest CB as a CCB so that he doesn't get exposed in the wide channels 1v1 against pacy attackers. That's how de Ligt should be used too. That's how Dias is used at City too. Botman at Newcastle too. You mask their weaknesses (pace) and maximize their strengths (anticipation, timing, communication, organizing, reading the game, pinning down opposition players, and, well, defending).
He is extremely good at winning physical duels despite his small frame, meanwhile outside of aerial duels Maguire is often turned inside out by defenders running at him do to his tank like movement.
Maguire reads the play terrible. He often acts proactively leaving open acres of space and completely pulls the entire defensive out line out of position.
Maguire has had severe problems with working with several team mates apart from Evans, which is why almost all our other cb's have looked significantly better in other partnerships.