Erik Ten Hag has been able to transform the team mentally, physically and tactically. He has also been able to help improve some of the players who have been performing poorly, players like Darlot, Rashford and Fernandez have been playing alot better in the past 4 games so are commend Ten Hag's efforts. Ronaldo's situation I believe is just for the main time and in a short time everything will fall in place and he will get back his position in the starting eleven.
Ronaldo is part of Ten Hag plans actually. But for some reason that makes Ronaldo want to leave Man United and he throws a tantrum to make his wish come true (he left the stadium when the match was not over in the pre season match), this makes Ten Hag choose to bench Ronaldo. After all Ten Hag is the boss and it's right that he should teach Ronaldo a lesson. If Ronaldo can do some introspection and showing good progress, I don't think there is any reason for Ten Hag not to choose Ronaldo as a starter.
Making Ronaldo an option not to be used as a starter is the best choice because when he becomes a starter it will mess up the plans or strategies of Ten Hag and eventually lead to defeat because Ronaldo's contribution at the beginning of the match could not work as a team well so there was no other choice but to put Ronaldo in the second half. In fact, Ronaldo cannot change his bad attitude so that he is currently not liked by many big clubs.