1. While they aren't currently accepting new API partners, that hasn't always been the case. There are quite a number of apps on both the iOS and Android platforms that are using the API, and some are very highly-regarded and widely preferred over the official apps.
2. Groundspeak is under no obligation to follow the "open" movement. If they feel it's in their best interest to keep the API proprietary and closed to new partners, I don't see why they shouldn't be able to.
As for the rest of your post, I wholeheartedly agree. I think there must have been some technical or legal reason that forced Groundspeak to prematurely kill off the old app and roll out this unfinished one, and they aren't at liberty to tell us the reason why.
Their best interest should be in listening to their premium members and adding features we need or they're going to see premium membership counts drop. I'm on iPhone but since they're retiring the classic app I'm retiring my fundage for premium unless I find an app that works similar to classic.