There may be a way of using the coinbase fee also in this calculation, but treated differently.  The coinbase fee primarily serves the emission and distribution functions, but also stimulates adoption in the early years.  It might be used as a way of amplifying the metric in the early years (when lack of adoption is a significant existential risk, and percentage growth is presumably higher) and then let this effect subside in later years by some form of multiplying by (Coinbase)^-1/2

Currently the cost per transaction, with the coinbase included, is often higher than the transacted amount.  Such transactions would not occur without the coinbase, so a way to accomodate for what this proposal would mean (because we would be unlikely to have any meaninful MAX_BLOCK_SIZE increases so long as coinbase transactions are the funding source for the network.

If I miner did somehow push the fees and blocksize up, that miner could then publish large blocks in an attempt to spam / DoS the network.  That's the only real threat, and it could be very costly and slow for the miner to accomplish.  Unlike the difficulty adjustment, which is bounded at 0.25X to 4X, the max block size adjustment could have a much tighter bound, like 10%, so that it would take months to double the max block size.

I think this is simple and straightforward enough that miners, developers, and bitcoin users can read it, understand it, and be OK with it.  I also think that it's safe long-term, and doesn't require human intervention once set up, regardless of how computer technology evolves (or fails to) over the next few decades.