I would guess in the half a million range.
New TLDs are coming out all the time now - which makes having to have a .com less important as consumers become accustomed to irregular domain names.
It'll actually have the opposite effect. Hundreds of domain name extensions are coming out, hundreds. That would give ground to terrible confusion. That in itself, could raise the .com domain name value.
Seriously, no one could remember 20 different domain name extensions like .desert, .coffee, its just too stupid..