It was my understanding that even then, Vishnu and Shiva are aspects of Brahman. It’s just that Brahman is so abstract and transcendent that you don’t really “worship”. If you view worship as a form of evocation, Brahman doesn’t really give you any attributes to focus on.
It’s like playing music to evoke a mood. Holding down every key of an organ can only really convey the idea of “EVERYTHING, undifferentiated”. It’s certainly a compelling idea, but it lacks nuance and texture. If you want to convey something more “useful”, you have to be more selective.
Krishna isn’t even really part of the Trimurti, he’s an avatar of Vishnu. You could draw parallels to Christ, “personal” is a crucial qualifier there.