I generally agree with this sentiment--the biggest value blockchains provide to decentralized applications is an inimitable 100%-replicated log (where inimitability is provided by PoW in these two cases). Once you have that, nodes in a decentralized application use it it to record their externally-visible state transitions, thereby giving each node a global, total ordering of all of the application's prior states. Then, the usefulness of one blockchain versus another is simply a function of how secure it is (i.e. how inimitable it is), how much bandwidth it offers (i.e. how much state can be recorded in a block), and how fast it can add new blocks.
This is the approach Blockstack DNS uses, for example (full disclosure: I'm its lead engineer). It's designed to be portable across blockchains, so we can migrate it to a more secure one if the one it's currently using proves insufficient.