T250033: SocialProfile prevents display of GlobalUserPage is the mentioned GlobalUserPage task

This is undesirable.

No, this is literally an intentional feature (see c115f5acfd61deb464f529821e61d5f291f2de79). The underlying logic is that with SocialProfile installed, a page like [[User:Foo]] is guaranteed to display something; thus red links in NS_USER don't really make much sense, especially if and when they point to a social profile page (which isn't world-editable even on a wiki that is otherwise open to anon contributions etc.).
I did consider a $wg variable toggle for this feature, but decided against it due to the aforementioned reason.

I think we should solve this by:
(1) Making the wiki user page the default user page;

That's literally the default without SocialProfile installed, though, yes? I'm not exactly sure why you'd want to do this, but I'm willing to review a patch which adds a $wg* variable of some kind to toggle this behavior, but it should default to social profiles being the default.

(2) Removing the SocialProfile user page header from the wiki user page.

I don't think this is a good idea for it'd hinder the discoverability of social profiles from wiki user pages and vice-versa. cc'ing @Isarra for design input.