Why weren't the XB-70's wingtips fixed in the down position?
Well, for starters, the drooped wing is not producing any lift in subsonic speeds. So, the flat area (the fixed part) should be significantly larger for producing the lift. This increases weight, which leads to a vicious cycle.

Another thing is the wing would panels would not clear the ground while the aircraft was on its landing gear. For testing the wing fold in ground, the aircraft had to be put on tall jackstands.



Image from keypublishing.com

As you noted, lengthening the landing gear enough to allow the wing droop would have been a big issue not the least because the aircraft weight was already increasing alarmingly (the weight had shot up from 483,000 lb to 537,000 lb).

Also, you lose the outboard elevons; now for getting the same control response the inboard control surfaces have to be increased in size, which has to be come either from wing or increase the wing area- it is again a mess.