There's a bit of a dichotomy with the transaction based approach. The wallet will need to both be aware of every single transaction the users make and need to restrict functionality to be dependent upon the third party. There can also other things that aren't dependent upon transactions.

Some of these reasons is why Mutiny offers a subscription. Right now it's strictly for donation but in the future it'll have features you don't get elsewhere in addition to hopefully offering one or two free lightning channel opens for paying users.