if they come up with a way to throw away all the X oldest blocks periodically then that could guarantee the blockchain never gets bigger than a certain size and then you could have block sizes as big as you wanted. 
That doesn't sound good, Quote
but you can have a look at MimbleWimble.
what's Mimble Wimble got to do with blockchain size?Quote
But still, there's no way BTC does a hard fork, unless something radical happens (SHA-256 collision, quantum computing etc.)
How about people get tired of a huge blockchain that's terabytes in size?