I think making Flash into Venom was a master stroke, a brilliant idea from Dan.
One of the problems with the classic Venom, love him or hate him, is that Eddie Brock, for all of the many, many pages devoted to him, was a fairly thin character. Nobody ever truly made him into an individual that you much cared about. So you were reading VENOM for Venom, not for Eddie.
But Flash has 50+ years of investment built into him. We’ve seen him in a bunch of different roles over the years–tormenter, friend, accused villain, romantic rival, amnesiac football coach–and all in context to Peter Parker. So making Flash into Venom gives that character a concrete hook into the world and life of Peter Parker in a way that was never possible with Brock. And especially after Guggenheim’s story about Flash’s service and the loss of his legs, he was in about as sympathetic a position as he’d ever been in from a reader point of view.