“Breuers I believe was the first American shul to start relying on kulos and doing Ma’ariv right after Mincha. I believe everyone else started when half of Monsey would pack into KAJ every evening.”
Davening minchah/marriv as a combo after shkiah has been done by Ashekenazim for quite possibly over a thousand years. The Rema davened minchah after shkiah, the Maharil davened minchah after shkiah, chassidim have continued to do so, etc. See siman 233.
The point is, it’s not a kulah just because it’s not your minhag. It also doesn’t retroactively become a kulah just because the M”B prefers that people not do it.
What does this have to do with davening pace anyway?