I was also in the same boat as you, but McTominay has started to turn me around. Ironically, van de Beek had a similar type role in that Ajax team. He wasn't a pure 10, not really an 8 either, and should be that guy that can do McTominay's job and offer a bit more technically.
But how many players have come and go, how many managers have come and go, and yet McTominay is still here, still getting considerable game time and contributing heavily to the team. That's not something that should be undervalued.
I think the key for me is that EtH finally sees what type of player he is. Mourinho, Ole etc seemed to want to mould him into a DM. I don't know if it was just because he was big, and you had the likes of Matic and Carrick at the club and managers thought they could be role models for him, but EtH actually understands that McTominay is not a conventional midfielder.
He's like a box-to-box No. 10, if that even makes sense. Does all his good work in both boxes, and doesn't influence the midfield like every other midfielder should. That's both a negative, but also something that makes him quite unique. It's also worth pointing out how late McTominay hit puberty and made his breakthrough, so could be the case that he is still to peak as a footballer.
I don't know what it is about him, but there are many others I would look to get rid of before him.