Superman and Batman have a good working relationship and show respect to one another, but there always comes a time where superman has to remind batman that he is an ant compared to him and he can beat him easily. For example, after tower of babel, Superman showed complete disrespect towards batman by demanding he take off his mask and talk to him face to face. He also threatened batman by saying, if he does not do it then he'll make him.
When things are all nice, superman will praise batman but if he gets angry or upset at him, all that goes out the window and he reminds him how weak and pathetic he is. He even goes so far as to insult the dark knight and remind him that he can do a much better job at stopping crime than him.
Now, what if batman from the main timeline decides to follow in devastator's footsteps and actually make a doomsday virus for himself? The virus would allow him to freely transform back and forth between his doomsday form and his human for,m. If you want an idea of how it would work, you can think of Davis Bloom from Smallville, the incredible hulk in the mcu or even the devastators transformation from the comic "Dark Knight metal: devastator".
With that said, how would superman treat and see batman if he knew that batman could become his physical equal in an instance and he couldn't just beat him up or swat him away like a fly whenever he wanted?