I don't suppose that we would have empty seats. I just don't presume that we would certainly fill a 40000+ ground, either. Perhaps we would. We just don't know.
There isn't a published waiting list for season tickets, so its hard to gauge demand. And demand is the only viable reason for expanding the ground. Image and prestige are excellent side effects of a successful expansion but they are not, in isolation, viable reasons to do it because (firstly) those things don't pay for the work and (secondly) we will gain neither if, it turns out, the seats don't get sold out.
My point was that whereas some clubs have built or been gifted very large stadiums and then had to come up with financially inefficient schemes to try and fill it, at least if we build Selhurst stand-by-stand we can test the extent of demand as we go.