There is a thought I've had. If BTC were to go past the moon and crush all fiat currency
go past moon? you mean the price or adoption? because price has nothing to do with what you are saying.
and also bitcoin is never going to crush, replace, or do anything else to fiat currencies and there are already multiple topics about that. government will never let it happen.
it would undergo far more transactions than it does now, more than SegWit could account for.
bitcoin like any other technology is evolving, and getting better. just like the first car that came out couldn't go faster than a walking old lady and now they go super fast!
and in the future
if it is needed it will change otherwise be replaced with something better.
Is there any way that BTC could become fast enough to function as a single or primary international currency through further updates yet to be determined without upping the block size to 1 TB per block someday?
a normal transaction is about 258 bytes
1 TB means 1e+12 bytes or 3,875,968,992 transactions in one block i seriously doubt that we can become that big ever,