Surely you would agree that there must be some SP level where the account can't use all if its vote power,

The very nature of a freemium system is that basic functionality must be free. Maybe that's "sponsored by @steemit" in the sense of resource usage, but it's not optional unless we're going to a pure pay-for-play model, which is not what anyone said they want, and is not something I believe will be practical in the long-term.

AFAIK, the RC system was designed on the basis of physical resource budgets (for example, that it is sustainable for the blockchain to grow at ____ MB per day, that ___ msec of CPU is allowable per block, etc.) and then allocating these resources between users such that they can't be exceeded in the aggregate. These are real resource limits upon which the survival of the system rests.

As I pointed out to @andrarchy before the fork, there's a real risk that what we discover under this system is that Steem never really worked in the first place. That seems to be where we're heading right now.