This is a bit "one megabyte should be enough for anyone", but...
A 64-bit flat address space allows up to 4.3ish billion times more space than a 32-bit address space. That's 17,179,869,184 GiB.
Obviously, the transition from 8 bits to 16 bits was pretty quick (I'm viewing things in a teen-during-the-80s way, ignoring all those mainframes and minis because they couldn't run a port of Elite). The transition from 16 bits to 32 bits took a bit longer, and 32 bits actually lasted quite a while.
Now we have 64 bits, would it be stupid to say "17,179,869,184 GiB should be enough for anyone"?
This is a programmers question because I really have the programming viewpoint in mind. Even if a computer some day has a seemingly infinite amount of working memory, that doesn't mean that applications will need to see a seemingly infinite flat address space.
Basically, can we programmers breath a sigh of relief and say "well, whatever integers or floats may do, at least I don't have to worry about my pointers growing any more?".