An article that I wrote recently for Aviation International News (AIN), where I was formerly the defense editor, touches upon the above question. The story is primarily about the forthcoming demise of JSTARS, but some of my discussion is relevant to the U-2:
https://www.ainonline.com/aviation-news/defense/2019-06-15/does-jstars-demise-spell-end-airborne-isr
However, the inherent flexibility of the Dragon Lady should keep it viable for a good few years to come!
Dragon Lady Today 
As a former U-2 pilot, I can attest to the fact that “Can’t satellites replace the U-2?” is one of the most frequently posed questions heard! And the two words “inherent flexibility” are the perfect two word answer. When satellite orbits are changed for a specific need — and they surely CAN be changed — that alteration comes at the cost of the loss of photographic access to innumerable targets that previously were available…not to mention the use of precious positioning fuel in the satellite.
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