If he chooses to stay at Man United, the pressure he will face will be even greater, therefore moving to West Ham is a good choice for his career.
Even though i think Harry Maguire got more sticks than he actually deserves, overall his time at Manchester United was a complete failure, he should have left a lot sooner than now. I find it weird when players decide to stay in their club, when it is obvious things aren't working out for them there and they prolly are not wanted.
Maguire will improve in West Ham, i am pretty certain about that, i think what he needs is just a fresh start, which he is going to get there. I think West Ham have done a pretty impressive transfer business thus far this summer, they have also brought in James Ward-Prowse from relegated Southampton, and Edson Alvarez from Ajax, though they lost Declan Rice to Arsenal but they got good money for him, and even if they lose Lucas Paqueta to City, they are also going to make over £80m in that transfer, so i think they are having a good summer.
Farewell to Harry Maguire he shouldn't have been a Manchester United player but it is good he has the exposure entirely. West Ham also wanted the Scottish player Tominey but he is ready to stay at Manchester United for another season maybe for another six months.
Good business, now they also want to lose Lucas Paqueta to. Manchester City, it's a good upgrade for the player but a big lose for the Hammers. They better sit out else they will be relegated just like Southampton and Leicester City.