Yeah, 64 is a nice round number (base 2), so it will probably end up being like that.
It's just such a huge number of teams, but I guess you can always just keep adding groups, and then the knock-out stage is just increased in one step only.
It might work actually
I don't think that the tournament will be expanded further, at least not for the next 50 years or so. 48 is a number that gives equal importance to both sides of the argument, that is those who argue that quality will get diluted and those who want more representation from around the globe. The main issue that we had with a 32-team setup was that it gave too much importance for the UEFA teams. 13 out of the 32 slots were allotted for the UEFA teams. It was justifiable in the 1990s. But now the quality of AFC and CAF teams have increased to such an extent that they can give a tough challenge to the UEFA nations. So in order to accommodate them, the tournament had to be expanded.