Why RJ is Not Commercial Maintained?
Your comment shows a misunderstanding of commercial vs. military use. Commercial aircraft are used at high intensity levels with, usually, a multiple flight cycles each day. Maintenance is a profit costing item and only those required inspections are accomplished.
Commercial aircraft do not undergo major upgrades every few years where most of the interior is removed, equipment and structure inspected and upgraded.
RJs are on a modification cycle that requires upgrades to current Block standards. This is a continuing cycle and the entire RJ fleet is NEVER the same Block version. For example, RAF RJs will be Block 11, I believe. USAF RJs delivered last year were Block 10s. Those in 2014 will probably be Block 14s.
This is NOT a commercial-type program.
If everyone is so concerned with using the -135 airframe, I am sure L-3 would modify any new, comparable airframe to a SIGINT platform. Of course, 40 years of working RFI and other mission-related issues would need to be worked from scratch. How many other RAF units, of the diminished number remaining, is MoD willing to cut to pay the enormous bill this would require.
One question I have with modern aircraft, especially those designed only for airliners, is how robust the basic structure is when you cut and add all the different bits to make an RJ-like aircraft? The -135 was very overbuilt and has had many, many mods involving holes, cheeks, noses, antennas, blisters, probes and other things added and removed all over the airframe. Will the composite aircraft of the future be able to take all this needed work without a complete (costly) redesign?