I think the original goal of service was to sell literature. Sure, they got recruits and that also lead to $$$ but even if they didn't get the recruits, they still had a steady flow of cash from the publishers who were buying the literature and then reselling, er, placing.
After that horrible decision to drop the set pricing to avoid taxes, they are left with the same model of using fs to move literature but they rely on voluntary contributions (again mostly from publishers) to provide their profit margin. However, folks may be donating the same amount they used to pay (maybe, probably not) but that was what, 15 years ago? Then, add in the fact that most of the growth is coming from poorer countries with less funds to "voluntarily" contribute and you've got the current cash crunch.
The control mechanism is obviously a huge part of fs and I think they later learned they could use it as a touchstone to measure everyone's worth to the corporation. The more hours you have, the more literature you sell, the more money the corporation makes. Top male salesmen could be promoted based on their production.
But, if you don't have the motive to move literature since selling literature doesn't have an acceptable margin anymore, it seems like you'd want to come up with another control mechanism with either busy work or legit work (such as in the case of the RBC).
Organized preaching will always have to be there in some form or fashion because besides refusing blood and not celebrating holidays, its the only thing most people know about JWs. You can't drop your signature trait. But, I do think they will change the methods to reduce the amount of literature moved by going to tracts or special campaigns.
There are a bunch of different senarios that I can picture where you keep the preaching work but leave it to pioneers (similar to what the Mormons do) or have everyone participate in special campaigns 4 times of the year with the rest devoted to something else. You could still keep the control no matter what the activity.
I don't think for one minute the GB would ever completely drop preaching. But, I do think something is going to have to change to reduce the amount of literature produced to improve profit margins.