Umm, how is it irrational to bank years on a large contract?
Sure the risk isn't huge, but it's still there. It's far from unusual to become more risk averse as there is more to lose, especially when you can do it in a way which doesn't really jeopardise future earnings. If you're on 50k a year and may not even have a contract next season you would tolerate far higher levels of risk.
If you've just won everything in a season, and have three years at 3mil+ locked in regardless of performance, it makes complete sense to go quiet for the first year or two and return to peak in time to negotiate the next contact. Granted you probably couldn't pull it off more than once, so even if Gilbert has a 2011 season next year, he's not going to get the same deal. Teams would also be inclined to link part of the contract to performance.
As an athlete continues to rack up achievements their legacy and post-career future become more influential. Being suspended at 35 isn't a huge thing as your time is numbered by then anyway, but if you're a big name you've lost all the decades ahead in which you could have milked your success (plus the greater emotional burden). So for Schleck, Evans, Wiggins, being a "Tour de France Winner" may have changed their outlook - not that the issues those three have faced could be down to (not) doping alone.
Contador is interesting as he probably has 5-6 good years left in him, two contracts after this one, ~15mil total. If he got done now then it's all over, so it again makes perfect sense to take a more steady approach for now if that's indeed what has happened. You have to be pretty fanatical to go for it year after year. It defined Armstrong. Contador had it but may never regain it. Nibali 2.0 seems to have found it. Froome has it if his talk of Tour domination is honest. Others seem to be susceptible to the emotional burden of it all (it's not just a simple financial decision). As such, I expect Andy to return in 2014 or 2015, but have doubts as to it being sustained for years. Frank seems more robust and able to instil confidence in the younger.