This is a ridiculous argument - which is essentially - if Southgate picks a player he must be the best available full stop and there's no point debating it- and then basing it on over the top reading of one game in which Maguire made a terrible mistake in possession to justify he's the best in possession. If he's making mistakes / can't hack it in games like Copenhagen, Scotland, North Macedonia (and getting away with the latter) it stands to reason there's a mistake in him when he's against France/Germany/Spain in an England team that tends to create little (and often has less of the ball) when we play Europe's elite still. He might be alright on the ball, but he is also ponderous, slow, not good in a high line, has a mistake in him in a team that can ill afford them, and has maintained a place out of sheer loyalty even when not playing for his club.
The argument that qualified people pick him so therefore they are right then falls down seeing as Erik Ten Hag jettisoned him to play a left back centre back for a while... before injuries he was basically their 5th choice centre back. So how would you reconcile an argument solely based on he gets picked so he's the best with him not being picked and an English left back playing ahead of him?
Perhaps Ten Hag wishes he needed Maguire's passing skills to make Kane and Bellingham thrive....
...but I think he has different priorities?