Basically agree with most everything you've said here. 
Blockchains are great for *global* double-spend. And the classes of concern where double spend are relevant are money and other forms of asset ownership. Bitcoin as money and [ethereum or whatever else] as "the other, arbitrarily complicayed complicated things you could own" makes sense to me. 
But I also think thst most "ownership" should be in-network in voluntary patchwork subnets, not "global" - so a lot of that also doesn't need blockchains.