I think McTominay may become a good player in time. It is easy to assume young players will develop their skills over time, minimise some of their weaknesses, some do, some don't. My record on predicting futures is far from perfect; I expected Januzaj to become an outstanding player, that hasn't happened yet.
I thought Scott performed very well against Liverpool last season and did a decent job against Chelsea and Sevilla. He was fine in the Benfica CL match, Bournemouth and Huddersfield in the PL, struggled against Palace. Good game against Swansea in the League Cup last season, not great against Bristol City. FA Cup games fairly ordinary. This season, played well against Southampton.
The aspect that confuses me is that in October 2017 McTominay signed a contract taking him up to summer 2021 with United having a one-year option, so it was hardly urgent that we make a decision, and this new deal will probably have minimal impact on his transfer value.
He should get more opportunities once Fellaini and Matic have departed. His height will be of more value when the squad isn't so packed with tall players.