> That would have the unfortunate effect of incentivizing miners to not completely fill blocks, because low fee marginal transactions could cost them money.

Look at the fee distribution. The vast majority of fee income comes from txs w/ fees near the LIFB. The blocks will be full... but I guess this would make Child Pays For Parent undesirable. CPFP would need a flag saying it is CPFP so that the parent fee isn't considered the LIFB.