With enough multisigs or mints, L2s can most definitely be "sufficiently" decentralized, particularly for payments; e-cash implementations like Cashu and Fedimint are good examples of this:
Having as many mints as possible to distribute risk and having key rotation as simple as possible is what is most important, which Cashu seems to be doing the best at at the moment. Fedimint has a lot of potential but is still quite technically expensive to implement due to the added complexity of using federations, which have worse tradeoffs than, say, partial multi-path payments on Cashu in some cases: