Note that the OP lists DEL-SFO, not SFO-DEL. I see a number of sub-US$2K fares on both UA and AC for DEL-SFO, for travel through March 2019. However, the cheapest expires today; the next-cheapest expires Monday.
SFO-DEL is considerably more expensive, and more restrictive (7 day minimum stay). However, the current lowest fare -- the $2650 that Rosalie Wang posted, plus or minus some taxes -- is valid through July 24.