Those added 30 minutes would've made a bad movie feel like an even longer slog. It wouldn't have fixed the ending of the movie, I still remember when Superman died someone let out a very audible "what the hell?" that made the whole theater laugh. The emotional impact of Superman's death... well there was none.
Back to The Flash, I think $100m is the floor. IMO The Flash has the best release date in June. Transformers doesn't look like it's breaking out. I have no idea what Disney is doing with Elemental. Spider-verse opens two weeks before so there's some breathing room. Indy 5 doesn't open until two weeks after and that right now is a huge wildcard.
The ingredients are there for a successful and leggy run, now the movie just has to deliver.