Very good logic. And I must begrudgingly agree. There are numerous APPs out there now, and some that have yet to be developed, that would not fit any of the proposed implementations for paying the APP makers.
However, while farmers offer a basic service to the network, APPs offer a similar (more complex) basic service to the network. Neither one can fulfil their purposes without the network - neither one has meaning if there is no SAFE Network.
Content is content and will be spread on SAFE, on the internet, in person, over the phone, over fax, any way that it can. Because it has value outside of the network. Vaults and APPs do not have this property.
APPs and the SAFE Network form a symbiant circle in that using APPs will most likely result in more payment to the network to publish content and therefore more rewards for farmers. The opposite is not true (putting more content onto the network does not lead to the creation or maintenance of APPs / putting more content onto the network does not necessarily lead to further content being put onto the network). EDIT: So APPs should indeed not be rewarded equally - they should only be rewarded as they benefit the network.
Therefore, while I can see your hesitation for supporting a one-size-supposed-to-fit-all reward model, I would argue that the implementation of this reward system is all that’s needed. But for APPs - it is, in fact, needed.
Disclaimer: I have proposed what I believe to be such a system here