For sake of example, suppose we have a marginal fee rate of 50 satoshis per byte.  At that rate reducing the size of the witness data by 1 byte is approximately equivalent from a miner and relayer's perspective as a replace by fee that increases the fee by 50 satoshis.  In both cases miners get an extra potential of 50 satoshis in revenue.

So in this sense replacing witness data with smaller witness data can pay for its own relay cost as much as RBF can simply by requiring that the smaller witness be smaller enough to cover the same RBF threshold.