This is similar to adaptive block sizing, which applies a penalty for generating blocks over the median size. The cost of the penalty is supposed to be mitigated by the value of the fees. My opinion is that this will not work well.
Such a mechanism doesn't work for this reason: miners generate their own blocks and include whatever tx they choose too, however the smaller the block, the less likely it is to ever be orphaned. If there was an overlay PoS network (like for MC2) you could try and enforce tx being included into blocks even at some level of reward penalty, but naturally miners will just include as many high fee transactions as possible without destroying any of their reward. Such a method of securing a blockchain is highly experimental and may not work for other incentives reasons (that's for the MC2 thread). As PoW stands now, miners are free to manipulate the transaction volume/fees volume in the network as they desire, and so subsidy penalties will not really work.
I suppose this could also cause voluntary transaction fee payment to dry up a little as well. If the network will pay the "fees" through inflation then miners might nnot be so choosy.
I think I mostly like the idea of a convertible subsidy which can even disappear if the network no longer needs it, but one that reappears when it is needed.
I appreciate the fact you guys are struggling through this decision. I think it could be one of the most important fundamental choices to be made.