I am not arguing that most people will use mouse and keyboard only. But nothing speaks against it and having the option to buy without controller makes sense to me. Even if it costs 70 Euros more with controller, why would I buy it with one? And I am surely not in the minority of having a controller already, and not everyone wants to use a Steam Controller anyway.

All I am saying is, that there will be an option to buy it without controller probably, which helps Valve keep the price down as possible. Which is a very important factor, so that will happen for sure.