Good luck to him, and I hope he will become a great player for us. Was a dream target for many, including me back in 2019 after his breakout season at Ajax.
However, I think the need for Branthwaite was bigger, and signing de Ligt only makes sense if we're offloading Maguire.
Both of them are organizer type CCBs with not a lot of pace and cannot really be deployed as wide, channel defenders IMO, like your van de Vens, Konatés, etc. Both of them are high value, great players for me but I think they'd make an awful pairing, as well as switching to a back 3 in possession with one of them forced to be a wide defender would likely often end in tears. Having two very similar CBs of this level, who cannot really be paired with each other doesn't make a lot of sense IMO, hence my preference for Branthwaite (and I think he's overall better with a higher ceiling anyway).
We only really have Yoro to do what I described in the current squad, but getting Branthwaite next year should give us serious quality and depth at CB, as well as tactical flexibility. Two arguably generational pacey and physical defenders as well in Jarrad and Leny, with de Ligt and Martínez would give us 4 genuine quality CBs.
De Ligt could prove to be a great addition but it's a smaller step forward than Branthwaite or another similar CB could've been. But like I said, it could easily be a good signing in the long term, especially for £42m. A 25 year old Maguire or Dias (another similar type) would easily be worth that, so de Ligt should turn out to be a bargain. But ten Hag not salvaging Maguire and making him a key player is one of his biggest mistakes IMO. Signing de Ligt basically confirms he never really intended to do that anyways. Or maybe the club and ETH both feel like Maguire's started to decline physically. We'll know more in 12 months.