Why do people think getting Ndidi means we're not going to try for a better passer in the future? A defensive midfielder that can pass very well is always preferable, but having a destroyer as a squad option would be great. For example, if Garner develops really well he's the exact type of player people want.
Currently Matic does not provide defensive prowess, nor does he provide great passing. Ndidi is a little worse on the ball than Matic, but his defensive ability is much better than Matic. If we had Pogba and Ndidi, all we would be missing is that passing midfielder as then our midfield would be complete. Complete because it's able to help the side defend, help the side's creativity, help the side set the right tempo, help start attacks, and help the side by being able to bail out players from losing possession.
Matic doesn't help solve anything. Ndidi does.
That is true of all aspects though not just technical ability, and with virtually every player you bring into a side you are compromising on some facet of the game - you don’t get a #6 that is both Pirlo and Makelele, a #8 that is both Gattuso and Xavi, or a #10 that is both Zidane and Vidal.
I think we need energy, physicality and mobility in midfield. So if we did go for a ‘passer’ in the holding role, we would require the other #8 to provide that.
This is how I look at things too in regards to a midfield trio. It doesn't matter how you accomplish it. What matters is that the midfield is complete. One combination was Juve circa the Pogba Pirlo Vidal era. Another combination was the Iniesta Busquets Xavi trio. There are different combinations to achieve it. Ndidi helps accomplish one or two of the midfield's tasks. The last component would then be a technically gifted player who can run things through their passing.
Matic is currently better than Ndidi in passing and technical ability. Doesn't mean much if he's mediocre at any part of the midfield's tasks. He's not a great defensive player. Not a great attacking player. He doesn't provide energy. He isn't the best player to give the ball to in order to retain possession. He's not the best passer like a Modric is.