The thing for me is that Frenkie very clearly excels at the main things that everyone else in our midfield struggles with. Those being, 1. a strong short game pass, and 2. the ability to dribble and carry the ball in central midfield.
1. He averages an exceptionally high amount of passes per 90 minutes, with an equally high completion percentage. He gets on the ball a lot, but more importantly he recycles and retains it. He's in the 97th percentile for attempted passes, and the 93rd for accurate passes. Casemiro, Eriksen, and Bruno are all the 20th(!) to 50th~ percentile range for accurate passes between them. Meaning they're all a bit sloppy and careless with the ball - we know this, they're all very high risk high reward players. Frenkie on the other hand treats the ball with caution, like a proper Ajax Cruijffian possession nerd.
2. This is probably the biggest flaw across the team - apart from maybe Rashford, Shaw, and Dalot(?) -- we don't have any good dribblers. It's a big issue when being pressed, and part of why players like Eriksen and Fernandes are a bit one dimensional in bigger stake matches. Frenkie is in the 95th percentile for this against all other midfielders in Europe.
Even if he just brings those two aspects to his game, he offers something massively that nobody else in our side does. And if it's not him, it has to be a midfielder like him. Similar strengths. Not offence to any Tielemans, Ward Prowse etc. type shouts. They won't offer the things we're missing.