Just following this thread, and thought I would chime in my .02 worth. I think you have a valid concern about the various models canabolizing the market, but I also think as someone pointed out, that it won't be all that bad.
75% of all projects will need a Sc FF35. A Sc FF35 rental might be 200$/day.
15% of all projects will need a Ep S35. An Ep S35 rental might be $500/day.
8% of all projects will need a Ep F35. An Ep F35 rental might be $1200/day.
2% of all project will need an Ep 645. An Ep 645 rental might be $3000/day (maybe even upwards of $4k/day?).
The factor of which projects will need which camera is based on the differences of the camera. The point is, the more niche that a camera fulfills (a smaller piece of the pie) people will be willing to pay an exponentially higher rental rate for the capabilities of that camera. I'm just speculating on the rental rates, but if a project deems a need for a 645, and there is only 2 in a thousand mile radius, and the going rate for both of those cameras is $5000/day, without any weekly discounts, etc. and that is what the 2% of the project requires.. they projects that can afford it, will.
I think its a mistake to think (fyi, you are correct in /your/ thinking) that just because the 645 can shoot S35, that people will be willing to pay for a 645 at $3k/day over an Sc F35. It doesn't make sense for the producer/DP.
I'm considering buying the Ep F35, taking this model into account. After reading through this thread, I think I'll buy an Sc S35 because at the price point, its a trival "add-on".
I send my R1 out about once every 3 weeks, on average for two to three days (a 3:20 ratio). No a super strong rental market in Utah, bit it pays for itself. Going to the Ep, I hope will give me a niche that will probably kill my rental frequency to 1:30, but will also give me higher-end clients who are willing to pay a premium for the extra lattitude, frame rate, etc.
I also think that the quality of service, and in particular, the span of services that you offer will also play a role. Ie, you DI cart. Sure, you can rent a Sc F35 for $200/day (and nothing else), and for some productions that is fine. But again, discriminating clients would rather pay $1200/day for a Sc F35 knowing that they get a top-rate DIT and his cart for onset dailies, color management, data processing and archival.