Yeah, and thinking on it now, I think that might be a partial reason for some of the reactions I've seen.

If this was a culmination of a story where we had, maybe at times even subtly, seen signs that Bruce's relationship with Selina is affecting him, then there would be so much more power here. Moments when Bruce perhaps gets distracted or isn't solving something because of spending time with Selina. Even some kind of a reaction from the city or something. Then it would feel like a logical conclusion that had prepared the reader for the inevitable.

However, what King's story has been how Bruce is even more effective now as he finally gets loving relationships, which I can't even, and Selina is so awesome that the duo are just unstoppable together. So in this situation for Selina decide by herself that happy Bruce can't be a good Batman does almost seem to be a general argument about heroes being happy as it removes the actual specific character and the impact on him from the analysis.