They can't find a replacement because they won't want to find a replacement and they still think about respecting Ronaldo because he is a great player for his team while that's wrong and they should start seeking.
In fact, they can't find any player because they already have Ronaldo on the team, I think the best thing he can do is to get retire and that's how he can force Portugal to find an alternative player because then they will understand they can't use Ronaldo anymore and they have to find another player instead of him.
It's not that simple, it's like finding another player of the same quality as if by magic... Cristiano Ronaldo is a legend, he's in the select group of the best players of all time, that's a rarity and maybe someone similar will appear again only in a few decades...
They don't replace him because there's no way to replace him... Only if they put an average player in his place... which isn't a good idea.
You cannot go with the legends players forever as those players will have to retire one day. Yes, players like Cristiano Ronaldo or Messi are stars but teams need to find equally good players so that once they retire, the vacuum is filled or there is no vacuum at all.
The teams who foresee the future will always try to find the replacements of the star players before there era comes to an end.
Even saying so, we cannot deny the impact of Cristiano Ronaldo on his football team and globally. It will not be easy to find players with the same skills, dedication and longevity he has achieved, compared to all the players in the history.
It won't be easy to find a player as per his calibre and it might indeed take some time before we witness another player with a similar level of talent and success.