Only a few clubs can afford Messi or Ronaldo also. Even if a big salary is not an issue, they are too old and they are not players you can count on for more than two years.
Indeed. Since they have a big salary, only a few clubs are interested to sign them. But it was in the past, I don't really know if there are still some clubs really interested to sign them right now. There are already top young players right now, Mbappe and Haaland are the perfect examples. Right now, I assume, top football clubs are more interested in Mbappe and Haaland than Messi or Ronaldo.
Sometimes as a backup option is an older player, but I don't believe Messi will agree to sit on the bench.
Sure, Messi or even Ronaldo isn't a typical player who wants to sit on the bench if they are not injured.
Considering them for a substitution player is impossible.