I don't know maybe you will turn out to be right about it and we will see Rudiger and Militao making a great duo next season. I would hope so actually otherwise they can need to spend a lot of money for another defender.
The best Real Madrid center defenders may be Alaba-Rudiger, Militao isn't strong enough and still needs more experience. But whatever the best duo, at least they have 3 quite strong center defenders right now. Alaba, Rudiger, and Militao will make Real Madrid defense to be better in the next season, I believe it.
When it comes to Tchouameni, I don't know if Real Madrid need a player like him now. He is a defensive midfielder and they already have Casemiro in that position. However they might still see it as an investment for the future of course. He is a promising player surely.
As long as the transfer fees are not so expensive, it is worth buying Tchouameni. He can be the substitution for Casemiro for the next season, and can be planned to be his sucessor. Real Madrid invests their future if they can sign Tchouameni