Why can't they replace the old pages with a link to the new page? Or otherwise remove the contents from the old site?
I tried adding some helpful links to the new Runescape wiki back when the split happened, and within a few hours Fandom had permabanned my decade-old account across their entire network.
Not a huge loss, although it still feels like a bitter ending after I spent years sprucing up a bunch of their wikis.
Usually attempts to advertise migration efforts on high visibility wikis away from Fandom will be deleted by Fandom staff.
That's considered vandalism of fandom, and probably rightly so.
Could you imagine if someone declared a successor to wikipedia and edited all the pages to redirect?
Sometimes you just have to put the effort into making the new better, and it's a hard long slog especially against a well funded incumbent.
But like all problems in PoE, PoE2 will fix it. ;)