I don't think Rashford would mind being benched.
Nico is a player who plays directly at Bilbao and wouldn't want to be benched at Barcelona. That's why he didn't transfer to Barcelona.
Rashford is a player who plays every aspect of the attack. He'll likely be Olmo's backup.
If he fits in, he'll be a starter right away when he gets the jersey. If he wasn't confident in himself, he'd go to the Saudi Arabian League instead of Barcelona.
I don't think Marcus Rashford would mind rotating with Dani Olmo. Barcelona is a huge team, and his willingness to embark on a new challenge in Spain after England shows us how competitive the player is. We all know he wants to fight for the right to play. I'm sure playing for Barcelona won't be easy, so he'll keep fighting. I think it's a good signing.
I also think he's an excellent purchase, I also think he's ready to play the kind of tough matches he was called up for, Barcelona is definitely a great team and being part of it is always prestigious, so he shouldn't give up, he can do great things
Perhaps you're right that Rashford wouldn't mind being a second-string player. But I'm sure Rashford wouldn't be able to endure being a substitute all the time. He was often used as a substitute at Manchester United, and eventually, he grew tired of it and decided to move to Barcelona. So, in my personal opinion, Rashford chose to move to Barcelona not because he wanted to be a substitute, but because he wanted to be a first-team player. However, Rashford knew that to break through to the first team, he had to demonstrate his ability and prove himself worthy of being a first-team player at Barcelona. If Rashford had chosen to join Barcelona just to be a second-string player, why would he have moved? He was a substitute at Manchester United, too.
So, don't not misunderstand Rashford. He's a young player with immense ambition. So, I'm sure his goal in moving to Barcelona was to regain his place. He no longer had any hope of becoming a starter at Manchester United. That's why Rashford chose to join Barcelona. Because he likely realizes that he still has a chance to become a first-team player at Barcelona. So, for now, we shouldn't immediately say that Rashford is willing to be a substitute. Being a substitute would certainly be very boring. Therefore, Rashford will definitely give his best performance with Barcelona.