I agree one should not go to the block chain only because it seems cool. Should correspond to a real requirement and should be a viable response to the problem.
And with that in mind, I think systems will keep being heterogeneous in terms of technologies. So parts will be on block chain, some internal, some on external infrastructure, others with another peer2peer techno… The intelligence, I think, is to discriminate with a clear view what is the best combination for a specific problem. So again I agree, interoperability is key here.
About sharding, I have just read a white paper on another distributed consensus algorithm which uses a DAG instead of a chain. Maybe interesting, I could find the name of it if you want, I just forgot by now… Anyway sharding as always will boost performances or not depending on the problem.
And by the way, funny to read you here instead of the high mountains’s of South France ;)