they can create a 17-200 F1.8....Just that it will be so big no one wnats to carry it.
FYI, 4/3 system has a 14-50 ( 28-100) F2 comming up...
Actually, F1.8 is very difficult to shoot with cos the DOF is so small.
I rather have clean ISO up to 12800 clean....lol, then all the normal lens of 3.5-4.5 f stops are as good to use in low light conditions.