I actually don't think Maguire's been bad at United and I still think he's a good player - but losing Maguire has allowed Leicester to play with a higher line because Evans and Sonyoncu isn't anywhere near as slow as a pairing as Evans and Maguire. With Ndidi to help out the defenders, they can keep a high line and they have 2 very good fullbacks to stretch the opposition - which really suits how Rodgers likes to operate.
Also you have to consider that Maguire was playing behind a much better midfield at Leicester than he was at United (weird to type that one out and be totally serious about it - but it's true). So even though Maguire not being pacey isn't so much of an issue at United, where they defend deeper than Leicester do this season, he's under more pressure defensively because that midfield is putting the United defense under more pressure.
I think he was overpriced, he's not worth a fee as high as his, but I understand why they signed him - he's a player that gives them leadership (ironically, Leicester still have a leader at the back in United's youth product Evans) and he'll be with them during his peak years. The defending for United last season was pretty dogshit, this season it's been noticeably better.
My post about him holding Leicester's defense back was more in jest than serious - I think ultimately Leicester's defensive success comes down to Evans and Sonyoncu being a better partnership for the system that Rodgers likes to use, more than the quality of Maguire. It was an excellent bit of business for Leicester, getting as much money as they did while they had a replacement ready to go.