There are two basic requirements for an implementation of stable vars:
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Variable access must be (amortised) constant time, not time linear in the size of the data it holds. (Otherwise, it will be a gigantic performance/cost foot gun.)
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It must be possible to reclaim stable memory no longer used, without irrecoverable fragmentation. (Otherwise it will be a space/cost trap for long-running apps.)
It is not obvious how to implement stable vars directly in stable memory with the current API such that these requirements are satisfied.
Also, keep in mind that we cannot afford changing the implementation of stable variables often, since every old version must be supported forever, and complexity of the system roughly increases quadratically with each change (and testable correctness worsens quadratically). In practice, I think that means we’ll only have one, at most two, more shots in production. Hence I’d be super-reluctant to introduce half-hearted solutions, even if it’s just a stop-gap measure.