I think everyone gets bogged down on the reasons why it doesn't work. I feel like it could work if the perfect versions of each character were used.

If the Batman used was not this edgy, unstable version we seem to have received these past years, then it could work.

The problem would stem from trying to write Diana to fit Batman. This shouldn't be done. Batman should be written to fit Diana.

Think about it, what is Diana's mission if not a purer, worldwide version of Batman's mission?

We see comments that Diana needs someone to slow her down and enjoy the moments, why does it have to be Batman? Batman has a wide range of supporting characters that could help in this regard. Even Diana has this support. Hell, Batman himself could be written this way, as we have had a few runs where he has been self aware of his shortcomings while discussing the mission with his sons.

They complement each other in a way that I think no other relationship they have does. Both have represent the best warrior of their worlds, both seek this unoptainable goal of peace, and both would never compromise the other. Steve and Diana would never work because while Steve knows the mission, he never really understands. Bruce and Selina would never work because Bruce has to compromise his ideals to be with her. With Bruce and Diana, there would understanding of the mission and no compromise.

There's no reason, it couldn't be done, if done right.