Interesting historical context here: way back in the day Mike Hearn advocated to use rate limiting to make nSequence based transaction replacement “work”.

What that means is while replace-by-fee allows a transaction to be replaced by a higher fee, nSequence replacement just lets you replace a transaction an essentially unlimited number of times without paying anything extra.

Of course, that was insane, so we got replace-by-fee instead.... but all these years later we're still adding rate limiting to cut down on any issues left over in edge cases.

Belt and suspenders!