While I think that Suhas’s example above is somewhat similar, I recently elaborated an example how introducing a replace-by-feerate scheme in addition to our current RBF rules would cause new substantial DOS vectors. While my example is based on a concrete proposal, I believe that it illustrates a general issue afflicting all schemes that do away with the absolute fee increase in replacements: mempool - What is the problem with the recent "One-Shot Replace-by-Fee-Rate" Proposal? - Bitcoin Stack Exchange
My example entails a negligible cost to the attacker while permitting continuous resubmission of essentially the same transaction cycle. The collection of transaction has a substantially larger total weight than the data that is ever up for inclusion in the blocks and hence severely underpays for relay to the detriment of the entire network’s bandwith-usage. I hope studying my write-up would help substantiate why transaction replacements must increase the total fees in the mempool.