I mean, they could move on to a new protagonist, but should they? No. At least, not right now. Plus, as it's been stated already, John's been in cryo multiple times and for extended periods. That, combined with him not aging normally anyway due to his genetic modifications, he's still got a plethora of juice left in the tank, as do the other Spartan IIs.
Sgt. Johnson was in his 70s at the start of the series, and he both looked, acted, and performed 30 years younger than that. And, since the Chief had been blasted with Forunner energy in H4, I'd say that he's got life and peak performance in spades. He's fine, and if/when Steve Downes decides to retire from the character, then there are surely other voice actors out there that could bring a similar gravitas and stoicism to Master Chief.
Also, side note: Master Chief has plenty of personality and character, it's just only been relegated to the novels and extended lore. However, his book personality has thankfully begun to filter into the games, starting with Halo 4. The man isn't really the super silent, kind of "one-note" solider that Bungie made him to be; he talks and emotes plenty.
