I do believe that Bitcoin can be used as a core backbone to link a variety of other more specialized (maybe even localized) systems via the use of sidechains and other technologies.
And this is a point that I have been trying to make (we don't need to put every single transaction in the world into one blockchain).
I agree. I didn't have the time to respond in your thread but I don't believe there is "one blockchain to rule them all" however even for those who believe Bitcoin can exists as an interchange between a diverse ecosystem of networks the block size will need to be raised. 1MB blocks prevent even infrequent access by users between the Bitcoin network and side networks without "trusted" intermediaries. If the backbone is centrally controlled then the supporting ecosystem is built on a foundation of sand.
We may see most of the overall transaction data occurring on side networks or way may see sidechains only used in niche applications where they provide a tangible benefit over Bitcoin "core". I don't know how it is going to play out there are advantages and disadvantages either way. How, when, by what method, and to what ultimate end the transaction capacity will be increased are all good questions.