What you've described here is different to what you have described previously. Taking a fee from every address, proportional to the amount of bitcoin being held on each address, and attributing it to future blocks is very different to taking a proportion of the transaction fees and attributing those to a future block. The former is sustainable, as you are always taking the same proportion of the same amount of bitcoin (21 million), creating a constant tail emission. The later is unsustainable, as you eventually reach a point where x% of the fees from the current block that you are locking for the future is smaller than the x% of fees previously locked which can now be claimed. With this arrangement you would eventually reach the point where the whole network is secured by fees alone (albeit with a small proportion of those feels delayed to a later block), which is the very thing you start out by saying is unsustainable.

You either have to create new coins and breach the 21 million limit, or you have to siphon coins from somewhere other than the block fees.