Memory and disk space will both expand to overflow existing technology. At the time Bill made his statement, few of us had ever used more than 64K bytes of memory. It was only after memory prices fell that the great explosion began.
I remember when 8K by 16 bits cost $1/bit. Compare that to today's costs and you can see why Bill assumed the growth of memory would be limited.
By the way, I have a 16K by 16 bit ferrite bead board framed in my living room as an example of the past example of memory technology.