I don't think there is any consensus that we actually want to increase the block size any further for now - that is the main reason why extension blocks have not been deployed.  The hard problem is not how to do it technically, but whether we need to do it or whether we should stick to what we have now with Segwit and scale on layer 2 instead.

While the block-size increase was one of the effects of Segwit, it was not the only and perhaps not the most important one.  Segwit also solves transaction malleability in an elegant way, which makes the implementation of layer-2 techniques (including Lightning) easier.