I think that the RAF Rivet Joints have been zero houred, so essentially have a full airframe fatigue life. As to the old, the design maybe, but the capability carried certainly isn't. Personally, I think seeing these arrive is a good news story as the ISTAR capability has been sorely lacking post the grounding of the Nimrod R1. Although Sentinel will have no doubt picked up some of the gap, a more capable platform with a more comprehensive mission system has been required for some time.
At the end of the day, the RC135 is an old design, but the same could be said of Sentry and even the 'brand new' Voyager. With all of these aircraft, I suspect that the platform is less of a consideration than the mission systems carried. Looking forwards to perhaps seeing one at an airshow at some point - a rather novel addition to the RAF line up; certainly has a face that only a mother could love!