Kobbie Mainoo is a Manchester United bound player and would end up being a legendary player for Manchester United just like the likes of Wayne Rooney, Roy Keane, Paul Scholes and Co's.
How would you know that, Kobbie Mainoo is just 18 years old, so how can you predict the long future he has ahead of him. Kobbie Mainoo is a brilliant footballer and Manchester United will definitely want to keep him for as long as possible, but who knows what will happen in the future, he might develop so well and at a time in his career, he could leave the club and go somewhere else.
It is true that Kobbie Mainoo is young player who is 18 years old, but no one can confirm or guarantee his future even though he has joined big team like Manchester United.
If look at the playing statistics that Kobbie Mainoo played with Manchester United, it is clear that it doesn't look very satisfactory, he has lot of experience playing with Manchester United and overall there is nothing impressive about his performance.
I completely agree with you that he could go to another team in the future because every player wants to have more experience in the hope of winning titles and also improving his career as player.
Moreover, the development and career of football player will really depend on the team he follows and the coach who is in control of that team, looking at Manchester United current condition, I doubt that Kobbie Mainoo will get everything in short time.
He has to go to another team if he really wants to have much better achievement and career than before.