Let me take a second and name a few: we have Ronaldo, Fernandes, Felix, Silva, Jota, Leao, and several other well established players in the top European leagues. I don't really see why Portugal shouldn't have a chance at all. It is not even all about Ronaldo anymore. That team can beat any team on a good day, but as you said they aren't the most stable team right now so they can also concede a couple of goals within a single game.
As I said before it really boils down to the form the teams will have at the World Cup, whether anyone gets a covid infection and can't play and so on and so forth. On paper right now I would say that Portugal might not be a favorite, but would you say that they are easy to beat?
Ronaldo is a class apart. He can't be compared with anyone else. Players such as Diogo Jota, Rafael Leão and André Silva are very talented, but none of them can replace Ronaldo if he is not there in the squad. Take out Ronaldo from the Portugal team, and I am sure that they will struggle to remain in the top-15 ranking. Players like Ronaldo makes life easier for the other players in his team. Because the opponents will be completely focused on him, and using this opportunity the other players can make attacking moves.
When it comes to existing players, Portugal has a better line-up than the previous year. Where they are strengthened by players who have stable appearances this season.
But if I let Cristiano Ronaldo not join the national team I think it will be a problem, especially in terms of mentality. His leadership is still very much needed to motivate other players. Yes, it can't be denied.