Here I fundamentally disagree: the reason is so obvious that I am surprised you yourself can't see it. Isn't it very easy to set the conditions under which Ronaldo should not be replaced? It is in my opinion and here is the evidence.

Ronaldo is by far the best goal scorer except for Fernandes who is only one goal behind him. For what reason should Portugal replace him now? I agree that they should have someone being prepared in the background who can take his position as soon as the time has come, but if he finishes a year as the top scorer of the national team, it would be stupid to leave him out of the team.
Well Ronaldo isn't in his prime anymore, literally by the next world cup tournament he wouldn't be a really good fit even if he may appear to be, age wise he's got passed 30 and then even his form most definitely will gradually reduce, this are just mare records he's been able to achieve over time and so there are other player if they are give the time he has played for the national team, may do even better but he wouldn't know except they are given a trial, so I'm of the opinion if they see a better replacement, they shouldn't hesitate than risking their chances at the world cup tournament, these records are there and made to be broken some day so they can't continue to keep references on it and not move forward.