[justin66]

The surveillance and EW electronics on the U-2 are kept up to date. You'd have to think long and hard about the value in developing an all-new successor to the airframe, rather than an incremental update to the U-2. The design is already very well suited to a very specific flight envelope. (as a practical matter, an all-new replacement made today targeting the "piloted and subsonic and fly as high as possible for a long time" would be required to have working stealth features and would be huge and cost a zillion dollars, and nobody involved wants to pay for that)

Regarding the aircraft comparison: the U-2 is surprisingly cheap to operate. It's brilliant. The SR-71 pretty much pegged the meter in terms of operating cost.