It's liability in a sense that Ronaldo defensive contribution is non existent.
Modern coaches are very tactical and expect all players to do defensive work so when aging player can't do so and run for 90 minutes, 1-2 players need to close that defensive gap.
This is why Ten Hag benched Ronaldo back then, he wanted Ronaldo to be "Super Sub" But Ronaldo don't want that
Ronaldo lethal finishing is his saving grace and if it's hit or miss then it's just liability.
I don't think it has to do with defenses.
It's more related to he can't press like when he was in prime and his involvement in build up of chances outside the box is less as well.
Ronaldo has become a player who's just about finishing whereas other players have to do most of the running, pressing, defending to create chances and feed the ball to Ronaldo to finish. Even in that case he's losing touch.
So, building a team around such player doesn't make sense.
A young, aspiring and decent player simply offers more to the team in this regard. For a manager, this provides tactical flexibility.
That's why I think Ronaldo has to fall back like Messi did and focus more on creating chances than scoring goals. But he won't do it.