Quantum Computers are advancing much faster in calc and storage capacity. In 4 years from now, QCs will have cracked the blockchain, and Bitcoin will be gone, or at least, residing on a QC.
your post is completely irrelevant here!
first of all we are talking about 4 years not 40 years. second of all even if QC becomes a thing available to public we can move to more complicated encryption to avoid them.
anyways i think as c789 pointed out, everybody has something more important in their minds rather than increase of blockchain size.
and maybe i am wrong but i think one of the reasons that some miners were against the block size increase was the similar reason.
also i don't think your 700GB is accurate because we don't know what will happen in the future. for now max block size is 1 and we don't know when it is going to be increased and even by then we don't know how much of it will be full. the number of unconfirmed transactions show that the current size is enough for the number of transactions and even if it was increased today the size of blocks will be the same unless someone starts spamming again.
so there are
too many variables at play that we can't be sure about the 700GB the only thing that you can say is that it will be somewhere between 240 and 700