The point is that no guarantee it will happen. Sure, he can work hard or whatever, but will he get enough minutes to get a spot in a national team or something else? Assuming he will gradually earn the starting spot means that either the players got injured or they got a drop-off in quality. Not sure if he's comfortable with that. A rotation player is necessary for a UCL run anyway, but from articles that we read on social media, he doesn't seem like the player who is content in that position. His situation reminds me of Elliot tbh.
Of course there's no guarantee when there are other players who are much better than him in the same position, and that means it will be very difficult for Rodrygo to secure a starting spot in the team. He's still quite young and there may still be plenty of opportunities for him if he can be patient and continue to improve his individual skills. But if Rodrygo's goal is to play regularly for the national team, perhaps considering other clubs is also a good idea, as long as he can guarantee a spot, as the national team itself has many players in the same position as him.