It sounds like you are saying that drive size will stay the same or even become less in the future. I definitely don't see that happening.
Cloud data can't replace data that's on your computer entirely.
I think what he's saying is that we won't ever get more than 512GB on mainstream laptops. Even most Windows laptops don't really go above 512, and it's much more common to see 256 as the only option offered.
Apple, for whatever reason, didn't add a 1TB SSD option for the rMB. They could have, but I guess it's to differentiate it from the 13" rMBP. I have a 1TB SSD on my rMBP now. We could see 512GB be the max on ultraportable laptops now and in the future.