In the 1559 case, we have two mempools but they aren’t intended to live side-by-side forever. The intent is that one eventually replaces the other. This is a bit different from other new transaction types where the intent is that they live side-by-side forever. If we imagine 1559 landing after 2711 and other new transaction types, I suspect 1559 will need to actually replace all transaction types with new transaction types that include the new 1559 gas semantics. For example, if we have transaction type 0 (legacy) and transaction type 1 (sponsored/batch/expiring transacitons) when 1559 lands, then 1559 would need to introduce two new types: 2 (legacy with 1559 semantics) and 3 (sponsored/batch/expiring with 1559 semantics).
Questions that I think we would need to answer to move forward:
Do we think that switching mempools is a common enough operation that it is worth trying to generalize a solution? Do we think that should be part of 2718, or should it be part of a separate EIP that defines a mechanism for dealing with pool transitions? Will we always want to go from one mempool to another, or are there situations where we may want multiple side-by-side mempools indefinitely?