Idk about Cunego, he just had one of his best years in a while, so I think he has a few more high quality one week stage races left in him. Unfortunately, I don't think he will win many more classics. He is a perfect example of what happens when someone is very good early. People put unrealistic expectations on them, and they rarely reach them.
For endurance athletes who are very good at a young age, it is well documented that they reach their peak earlier than others. The thing that is on Sagan's side however is that while the first thing to decline is explosiveness/sprint power, endurance increases with age due to accumulated training. This is one of the reasons you see guys like Jalabert, Kelly, and Hushovd to an extent getting better at stage races when they are older. It's a bit more apparent in running, as the normal progression was to move up in distance as one aged(although this whole theory has kind of been challenged the past couple of years).
Also on the flip side, those who aren't as good at an early age tend to maintain a high fitness for much longer than people expect, which is why people feel Cadel will be ok for a bit longer. I think Purito falls under this classification, as he wasn't pushed too hard for results till he became a leader at katusha