I should hope that’s not the explicit goal, but you don’t achieve it here; GST1 would still work.
Now that I think about it, you are right; the refunds would be still able to cancel out the portion of gas usage that is new-storage-fills. The refund rule would have to be more restrictive (refund only if 0 = original = new != current) to solve that issue.
You would still have block size variance because you have refunds.
But this is not true, because the key invariant that remains is that gas spent on execution would not go above the gaslimit. Every 15000 gas refund would be matched by 15000 gas spent on filling a new storage slot.
Based on the text I think you got the subtraction backward but it’s problematic either way.
I don’t think it’s backward! You refund only if you cleared more storage slots than you fill.
Another thing worth considering is that clearing storage is going to be less useful in the future, if we are implementing either weak statelessness or state expiry. In fact, truly clearing storage would not even be possible; you would need to leave a stub to show that the value is zero, as opposed to the slot being not-yet-edited and the value needing to be dug up from the past. So in the longer term, having less good incentives to clear storage is not even such a useful thing.