He's not first choice ahead of Casemiro. Our first choice is clearly Casemiro-Mainoo-Bruno. However McTominay is next in line and is currently ahead of Eriksen, Amrabat and Mount, so he's first into the team whenever any of the main three aren't available or are rested.
I've always been one of McTominay's biggest critics, but I do try to be fair about it. For Ole, based on the options he had available Scott was sadly the best of a bad bunch towards the end (well, Fred was the best but Scott was the best option for the second spot). Matic and Pogba were clearly finished by that time, and for all of Scott's limitations the team was more solid with him in it than them. The issue was more that Ole never seemed to prioritise strengthening that midfield to push him down the pecking order.
ETH has completely changed the midfield and tried to sell McTominay, but it really hasn't worked out. He bought Eriksen who is pretty much finished, bought Casemiro who looks almost as finished, bought Mount who has spent the entire season injured and is probably the wrong type of player, loaned in Amrabat who has struggled, and sold Fred who almost certainly would have been still our best option for that third spot next to Mainoo and Bruno. Due to all that McTominay almost got that spot by default for a large part of the season, and even now that the other options are back none of them have really stepped up to deserve the spot ahead of him. I do think ETH has prioritised him a bit too much, but at the same time I have to admit none of the other options are making it easy.