Sentinel didn't wait 3 years after 2008, the first deployment was in the same year...
Regrettably the sole reason for its obsolescence was SDSR 2010. There was a very simple paperwork solution to the (overstated) weight issue at that point and Raytheon had even scoped an EO sensor and underwing hard points (Globaleye is the same airframe). Once SDSR pulled the rug from under its feet, development was limited and penny packets were thrown at it to keep it running when the servers became no longer supportable. The original servers specified as late in the programme as possible and at the time were the highest spec items cleared for airborne access to TS info. Regrettably the pace of computer technology moves much faster than the CIS people that clear hardware for such uses......
We seem to have managed to keep it constantly deployed on operations since 2008, other than a short break to allow the crews to recuperate, which seems to suggest there is a need for its data. However, we do have to cut our suit according to our cloth and nowadays we can only afford sackcloth - I fully accept that the need for Poseidon and Wedgetail is far greater than wide area battlefield surveillance....I just hope that the politicians begin to reign in their ambitions overseas to match!