For a big city theater, there are always customers. But take a more rural cinema, have 60% of the moviegoer population base convert to MoviePass... for the theater, it may see a slight uptick because it isn't that expensive for the users and the cinema still gets the full ticket price. Then imagine MoviePass closing shop - are the 60%, hooked to 10$ or whatever a month while generating twice that in ticket sales, going to come back once MoviePass isn't around any more? There will be a sharp noticeable drop, and this is what scares theater managers. Hell even if it's just 100 people using MoviePass that's $2k in ticket sales a month (given 10$/ticket and 2 movies a month per person), and another $2k a month in sales from soda, popcorn, food and merch.

Especially as outfitting/retrofitting a theater with the newest advancements in dozens-channel audio, 8K 3D digital projections, the newest anti-piracy crap the studios force upon the managers and other stuff is a huge upfront capital investment... I would be scared shitless if I had to do such an upgrade because the customers demand it but I knew at the same time that a lot of the customer base might simply drop off at once due to circumstances way outside of my control.