If it is not worth it (presumably because most of the bandwidth will just be unused) then the ISPs should just bump everyone up to gigabit or whatever their end

If it is not worth it (presumably because most of the bandwidth will just be unused) then the ISPs should just bump everyone up to gigabit or whatever their end-user links support. Few people will actually use the extra bandwidth so there will not be much aggregate impact on the ISP. (This holds true until the next leap in bandwidth-requiring applications comes along.)