The Amgen Tour of California was perfect for Sagan, in terms of weather, attitude, skillset and California style of reportage. I actually think he is "bummed out" (as they say) that TOC is off the calendar for a year (or perhaps more.)
I have no idea how this affects his outlook towards retirement, but this has got to be an upcoming season he is not as excited about.
If they were having TOC, I'm not sure he would ride the Giro, it is not exactly his cup of tea, more of a "Bike Racer Bucket List" type of thing.