My take:
Dave Brailsford and Shane Sutton have already stated that Sky have broad ambitions for the future. They want to retain the TdF yellow next year, but they also want to go for the slam of all three grand tours and do much better in the spring classics. You can't do that with one nine-man squad, no matter how good, how versatile, that team is, no more than eleven men alone could achieve a league/cup/CL treble (for example). Contador was the last
man to win two out of three in the same year (Giro & Vuelta in 2008) but whether he was clean or not....
To do it really, undisputably clean, you need strength in depth, and expertise in depth. The first two seasons were a painful learning curve at times for them (who knows how Wiggo might have done in last year's Tour had he not broken his collarbone), but the masterplan has clearly been working very nicely indeed- from the lower two spots on the podium at the Vuelta to the top two at the Tour.
Cav's focus this year has been on the Olympics, and his training, his season to date and the way he handled certain stages on the Tour, have all been with Saturday in mind. The stages he won in the last three weeks were obviously important, but a secondary concern (for this season only!). Sagan has already been compared to Eddy M in terms of all-round ability (or at least potential), and he's definitely a major threat to future green jersey ambitions, but Cav has already shown he's become a more rounded and stronger overall rider than previous years. He nearly won the points competition on the Giro, had a couple of minor wins, and he certainly wasn't afraid of working for the team on the Tour. Yes, I have no doubt that OPQS, GreenEdge et al would love to add him to their rosters, perhaps they can offer him certain advantages (Garmin and QPQS strike me as the best bets if he wanted to become a Classics challenger, for example), but can any other team offer him what Sky can in terms of the complete package? I find it hard to believe that had it not been an Olympic year (or if the road race had been more helpfully scheduled) that Sky would not have targeted green as well as yellow. The leadout train seemed to do the trick on the occasions where it was deployed.
Same applies to Chris, Edvald, Geraint....if they jump ship, it might work out very well, but they could end up being the big fish in a small, stagnant pond. Sky might be aiming for domination as total as Schumacher and Ferrari's in F1 was for the best part of a decade, but there's a big difference- Schumi's team mates generally got the second step on the podium, time after time. In pro racing there's rather more opportunity to spread the top step around a bit. If if anyone can keep that much talent happy, it's Dave Brailsford.