Your confusing two issues here.
There are not many players that are versatile. If you want to train someone to be versatile, then he needs to have specific qualities. Lukaku has qualities which make him only suited to a Centre-Forward Role.
Let me give you an example. Lukaku is very Drogba-like.. Strength, good finishing ability, can hold the ball up etc. He does not have the pace nor flair to do an effective job on the wing. Neither does he have the vision, awareness or passing to play in the hole.
Notice some of the intangibles I listed. You cant really increase his pace or instill flair in him. Neither can you do much to improve his vision or awareness.
Therefore, due to this he is not a player who can be versatile. Even if it is a great quality to have I can list you many top players who are not versatile. Didier Drogba for starters. They are players specificially suited to play a certain position where they excel. Any other position and they are pants. Not entirely their fault
Plus match experience only works if you ware being utilized in the correct way. According to your logic, Manchester United should play Welbeck in any role.. even as a Centre-Back just so that he can be on the pitch because according to you thats better than sitting on the bench
What do you learn sitting on the bench then? Explain that to me.