Well...,
256 Tbyte Virtual Adressspace should be enough just like 640 Kbyte of Ram is enough for everyone.
If i want to map my single 16 Tbyte HDD into my VA i already use 2^44 Bits.
I'm not talking about any special hardware only a hand full of the biggest companies world wide have. I'm talking about my single Disk usecase. And with ~600€ for a single DIsk this usecase is affordable for Homeusers.
Imagine someone wants a Raid 6 with 4 of those disks... already 2^46 Bits VA used.
Imagine an application which wants to map 16 Harddisks with 16 Tbyte each into a single VA...
Then you will find out, you already passed the limits.
I agree, that in most usecases today it is not necessary and it should stay disabled by default.
An additional Level of paging can cost some power/performance and if properly designed maybe 4 to 8 KByte of Ram....
But if i could use 57 Bits VA by today i would have a lot less headache.