But the "under 18" landing page doesn't actually block anyone, including those under 18 from continuing on with the site, so there's really no incentive not do

But the "under 18" landing page doesn't actually block anyone, including those under 18 from continuing on with the site, so there's really no incentive not do it. If they move to a guaranteed to be filtered tld, they'd explicitly be asking for less traffic, which seems unlikely to me.