Sounds like they are leaving the site up but not writing new content? That could actually be worse than just taking the site down as it will quickly grow outdated.
Looks like they won't write new content themselves, but will have a system for keeping content up to date
I've long avoided w3schools, but a lot of comments on various HN threads suggest the site has improved a lot over the past few years. If true, this makes me less worried about the future of MDN.
Plus there's other reference sites I find myself using from time to time, for instance Can I Use[1] (which has a Patreon) and QuirksMode[2] (supported by donations). Monocultures, in my view, are not good for the environment - and that includes coding reference sites.
[1] - https://caniuse.com/