It's not gone yet, for at least two reasons.
One is that it still has the eyeballs of most if not all of the users of the new forum, which defines itself in opposition to MB (in the interests of bonhomie, let's refrain from comment on whether anxiety about activity on a rival online message board is or isn't laughable in persons over the age of 14).
The other is its digital footprint, which remains considerable.
SEO baby. You can't find the new forum via google, you can only go through MB. So MB's potential for recruiting new users is greater by orders of magnitude than that of the new forum.
Btw are there other sites where the MB regulars have migrated?