What we're saying isn't don't build E tickets; just that big theater productions are, in some ways, more helpful at reallocating existing park capacity than new rides are.
When MMRR opens, it will help to a degree with the other lines, but that will be more than negated by the large number of people that will be drawn to the park to experience the shiny new ride. And since any new ride (no matter what it is) can't handle that demand all by itself, the massive crowd of people will inevitably spread into other lines too.
BUT a show in the Fantasyland Theater is all upside, taking thousands of people off of the midway, but it's also not the sort of thing that people are going to travel to the park specifically to experience. Much in the same way that after awhile no one was coming to the park specifically to watch Soundsational, but Soundsational could still take a little bit of heat off of the ride lines for awhile.
Part of the reason WDW in general and DHS in particular has all of those shows even though they're older than dirt is that they effectively take people off of midways and out of ride lines. No one's paying $170 to go to DHS to watch Indiana Jones Stunt Spectacular, true, but thousands of people watching that means there are thousands less in front of you for ROTR. Which is why it's really a shame that apart from Fantasyland Theater and Hyperion, Disneyland just doesn't do these sorts of shows. Sure, they'd have to change them out (a win for the consumer but probably annoying for DLR, hence why they don't happen much), but if DLR had a number of big production shows going every day in the afternoon, all of them soaking up thousands, you'd feel a difference in crowding level.
So both new rides and big shows are helpful for crowds in different ways, even if they have different levels of drawing power.