He's been tried in a high line as a part of a pressing team time and again with no success to put it mildly. The fact that he has performed admirably in a low block, which is the only scenario that he's properly comfortable in, doesn't magically erase that and neither is that an untested hypothetical. When Maguire has to cover a lot of space in front of him, he doesn't really know what to do and often misjudges the situation (either not trusting himself enough and dropping deeper, allowing the attacker free space or trusting himself too much and flying in aggressively before getting outplayed), which is what lead to all that Maguire memes.