Mark Cavendish is bright, and a realist. Professional cycling is about making money, and to do this he needs wins. Sprinters don't last as long as rouleurs; and need continued good seasons, which equal as much money as possible, until they fade. Given Sky's target of winning all 3 Grand Tours there wouldn't be a lot of support left for a lead out train. All the new Sky signings so far are climbers - domesiques for the Tours.
Specialized missed Cavendish and his high profile, and have big money to get him back. Remember the stories that Specialized were trying to supply Sky in 2013?
David Brailsford is a pragmatist (well, mostly, maybe not so much about doping!). Mark Cavendish doesn't fit into the Big Plan; but it was useful to have him with Sky in the Olympic year. If Mark does want to ride the track at Rio, and can commit to the build up regime, I can't see him being excluded - as long as, like anyone, he can "make the numbers".