Yes, given line length and ticket prices, most people actually don’t do double features these days. Some do, certainly, but it isn’t a strong thing to bet on. Of course, its also often stated that if you cut ticket prices in half you would more than double ticket sales. Economics is hard! Let’s go shopping! (yes, I realize the irony there)
But Steve makes another good point. Marvel has no real reason to move their date, because at this point, no matter what DC/WB film apologists might want to argue about quality, Disney/Marvel is certainly winning the war in the popular mindscape. If you’re really going to try to play chicken on this, Marvel is likely to win weekend one purely based on reputation, regardless of how good either film actually turns out being. The MCU is a Juggernaut right now, and the DCCU is just starting out and has questionable success at best. If I were Warner, I’d move and look for a weekend where I was opening up against a poorly tracking chick flick or something… just because I want every advantage I can get. Disney isn’t worried about that right now.
That said, it’s a stupid argument anyway, because we’re 22 months out, and given the production problems films typically have the chances that dates might slide (particularly in Warner’s case) are more likely than not.