The only truth in this is that he's too slow to be a wide CB, but I was talking about the CCB role. Also, Martínez's physical profile is also the reason he can't play wide in a back 3, just like it is for Maguire and de Ligt. As a CCB, he's very valuable when we have possession. Defensively, though, he's just not a great CB and Maguire and de Ligt are both way better.
The rest of your post about Maguire is just not true.
He might be one of, if not the best passer CB around, but when we don't have the ball, his deficiencies are impossible to ignore if he's to be deployed as a wide CB. He cannot cover big spaces quickly.
Some players around him won't be able to, either: Maguire/DeLigt in the CCB + Ugarte as one of the CMs.
So it's important that the LCB meets the requirements in this regard. Shaw might very well be a better fit there due to him still being very fast and a bit taller than Martínez, and we won't lose out on much in possession either, as he's an excellent ball progressor, in some angles / situations he's even better at it, than Martínez, and we saw some of it yesterday. Shaw also reads the game very well as an LCB and I've been impressed by him there as he seems to be very aware as to what he needs to do and when he needs to do it.
Someone like Branthwaite is probably a much better option for the system we've just switched to under Amorim. He can control and cover the wide channels much better than Martínez, and that makes the CCB's work much easier too. It's how top teams who use a "slow CB" in the centre set up as well, with two fast space-eaters on each side of them: Walker, Akanji, Aké and Gvardiol does it for Dias at City, for example.
Having Yoro and Branthwaite next to Maguire / De Ligt is an excellent back three. Having Martínez there will mean we are very vulnerable against transitions, especially against pacey attackers.