In my personal opinion these airliners will not have much life left. The 707/720 came in an era where fuel was no problem and these first generation jets where fuel-burners, in a few words true camels with fuel in is back. Now this doesn't mean that they can be realiable but they are beeing overpassed by bigger airplanes that consume much less fuel then these. Even in cargo operations we start to see first generation wide bodies like the A300, B. 757 and B. 767-200 ready to make money. My personal feeling is that unfortunetly a few more years we will stop seeing comercial 707/720 flying, but relax we still have military 707's and their military version KC-135 flying around for many years. Don't forget for example that USAF has more than 500 KC-135B's ( that where re-engined with ex: airliners B.707/720's and their tails also replace by ex: 707/720 airliners ) and KC-135R ( these a much more complex program that replace the old turbojet water injection by CFM-56's engines ) and also we have the E-3's Sentries from USAF/NATO/RAF/Saudi AF/L'Armee de L'Air, we still have the E-8 TACAMOS from USNavy, the EC-18's and some tankers from other air forces, these true 707's modified to tankers like Italian Air Force with 4 ex: TAP 707's, Spanish Air Force also with some ex: TWA 707's, Brazilian Air Force with some ex: VARIG 707's, Venezuela, Chile. So there some plenty 707 or 707 variations still flying around!!!
CV990, the Maserati of the skies!