It's funny but my interest in Momentum vaporized at the exact same moment I looked at that price chart! What are the odds.
Seriously though I don't know whose idea this was but I have a feeling it's going to fall flat on its face. People are accepting the subscription model cautiously and gingerly. I think they generally like it, but only for large things (like a full DAW, an entire range of plugins or a photography processing suite). People are becoming weary of adding all these little "extras" to their monthly outgoings unless there's a good reason for them. The only decent plan here is the $9.99/month one, and that's a significant monthly payment in the context of all of your other monthly payments. I'd have to have a damn clear need for an app like this in order to justify paying monthly for it. If it were a modest one off payment I might buy it on the basis that it might come in handy "every now and then" - by pricing it as a monthly subscription they're restricting their market to people who really believe they're going to be using the thing on a daily basis, and I'm not even sure how many people that would be. Plus, you're going to get a lot of people who subscribe and then cancel after a month after realizing they hardly use it.
So yep I'm all for an app like this and good luck to it, however my view is that it's not a good match for a subscription model.