RC-135 Accidents
Since most posters do not know the airframe history I will assume cannot comment on my comments are only general in nature and not specifically directed at the RC-135 jets.
There have been no RC-135 accidents that were not attributable to anything other than pilot error, with one exception. The RC-135E, Rivet Amber, disappeared in a flight between Shemya and Eielson AFBs without a cause every being identified. This was a one-off specially modified program, lost many years ago, and no relation to today's Rivet Joints.
RC airframes are not only re-worked but, in many areas, are new over the years. All -135 wing skins were replaced in the '80s-'90s; the engine pylons were replaced when the F108s were hung, along with landing gear upgrades. RC-135 fuselage skins are replaced where necessary and the addition of external antennas, cheeks, extended nose, internal equipment results in many components being replaced to accommodate the heavier equipment and airframe changes.
You cannot compare this mod to the Nimrod upgrade to the MRA4. From my sources at RAF and the company the MRA4 conversion was costing a huge amount of money as each airframe upgrade was actually an individual custom-fitted kit, as the original airframes were not to a standard build. The Airseeker original airframes were near the last of 820 -135s built and are pretty standardized. There are now 20 conversions to Rivet Joint standard and others to the various RC-135S, U, and X models. Quite a bit more experience than four attempted MRA4s.
I'm am not familiar with the official certification hoops within MoD but the contracts would not have been signed if there were a problem.
And, back to the original question: there is still no correlation between the latest KC-135R accident in the AOR and the airworthiness of the RC-135.