Matt Birchler published a video earlier today on Tweetbot 6 and it's move to a subscription model. While I get his argument about updates needing to happen to make a subscription worthwhile, I don't agree with it.
Never purchase software based on future features. If it doesn't have the features you need to justify the purchase today, then it's not worth it. No matter how good the intentions are of the developers, they may never get to an update that adds your pet feature so don't expect it.
Look at what the app has, and purchase it if it's current features justify the cost. If they don't move on.