PatrickDC said: "Ok, I’ll ask: Why would a Wednesday matinee be more or less well attended than a Thursday matinee?"
Wednesday matinees are more standard. Having lived in areas outside of NYC, it is more obvious. My hometown runs a "shopper's special" bus trip to NYC only on Wednesdays, and mentions catching a Broadway matinee as one of the reasons to go. And when I take the NJTransit into NYC, parking is hell, and I even ended up going into the paid lot, and the attendant said, you have to get here early on a Wednesday with the matinee crowd, and this is like 2 hours outside of NYC, and the parking attendant knows Wednesday matinees are a thing (although, obviously, you could take the train in on a Thursday).
So, while Phantom does get to own Thursday afternoon, and anyone wanting to see a show then has to see Phantom or nothing, it is also detaching itself from a lot of mechanisms that bring the people nearby into the city and just the knowledge people have that Broadway shows have Wednesday matinees.