Who the hell IS Pico anyway?
Anyway, while I do agree Snake has a chance of not returning, I disagree with the notion that he has the highest chance of it. There are other characters with a higher risk of it. Also, if Snake goes so does Sonic. Snake leaving would cause as much dissent as Sonic leaving, as he was actually liked and competitively viable, he was also a very interesting and unique character.
However, as Smash Chu said, sometimes guests don't return under the notion that they are guests, so it COULD happen, and people should be prepared for it, however I wouldn't put money down either way. Lucas, Falco and even Ike leave before Snake does imo. Especially if Sonic returns.