If Cristiano Ronaldo was given the option to play for Real Madrid again when he left Manchester United, then he would not have accepted to play in the Saudi League. If Benzema had not started to suffer so many injuries, he would have stayed at Real Madrid. All these players you mentioned are in the Saudi League because they did not have the chance to play for the big teams in Europe on a regular basis, so they preferred to go to the Saudi League because they can earn a lot of money. But they may have ruined their careers. Because I highly doubt that after their contract ends, the Saudis will give them the same contract, their salary will be lower in the new contract and the reason is very simple:
When a player leaves a European team for Saudi Arabia, the Saudis become dependent on that player, so they make big offers like a 3-year contract with high salaries
But when that player is already in the Saudi league and the 3-year contract ends, the market value of that player is no longer worth anything, so the Saudis will set their terms, they may set very low salaries because they know that those players will never play for big European teams again
That is why smart players only go to play in that league when they want to retire
It makes sense what you say, if given the choice of returning to Real Madrid he would have chosen it over making a contract with Al-nassr, but what can be said, the reality does not work that way, when Al-nassr made a high even very high offer to Cristiano Ronaldo that was one of the reasons he accepted it, maybe after the contract with Al-nassr expires and there is no new contract extension, Cristiano Ronaldo will decide to retire, although i doubt it, it seems that Cristiano Ronaldo is still interested and waiting to play in the 2026 World Cup.