One difference with the Monero model is that the amount of the fee is not a factor of the penalty, nor is size of the coinbase reward.  The penalty is not multiplied by the block reward.  Additive, not multiplicative.

I'd missed that detail at first pass because it seemed impossible for anyone to advocate such a function, when on its face it seems to fail the 'future-proof' test.  

Without knowing what the fees will be in the future, how can the penalty be the same for every transaction?  Fee amount tends to adjust with how much milk and bread a bitcoin can purchase.  The penalty may thus become overly burdensome (if bitcoin value increases) or meaningless (if it falls).
We can know the coinbase reward, but we should not always expect that to be 99% of the total as it is now.  This will matter later.

The advantage of the Monero multiplicative method is that it does not have to guess, it works at all fee levels, block rewards, valuation, etc.