I don't really get the whole argument. Pointless. Firstly, Milner isn't a starting footballer for a top club over the long term. Never really has been, he's a utility man, whereas we never judge McTominay in that vein.
Secondly, who cares if he is or isn't as good as Jordan Henderson. He's probably better than McTominay especially in passing but who cares, Scott is a good age and has his own attributes, especially getting forward. It's no standard by which to judge a player.
The main thing we have to get is a manager that can appraise players and understand whether they can contribute to a winning squad. Not everybody has to be this world beating superstar. If anything the only comparison is probably that like Milner McT is going to get bugger all credit as it feels as if he's always viewed with suspicion by a large section of fans despite managers valuing them.
McTominay is going to be going absolutely nowhere. His role may change over the coming seasons. I could see him dropping down the pecking order, I could see him playing different roles, but it is near certain he won't be leaving. Cheap to keep around, works hard, has potential adaptability - no manager is stupid enough to discount him from a squad.