There is a good way to defend the Musar Movement of Reb Israel Salanter. This way would have to start with an unspoken premise of all authentic Lithuanian yeshivas that the rishonim are always right. The achronim however can and do makes mistakes, This premise is based on objective facts.
But it is not widely known. The Middle Ages fell out of favor in the secular world.
The thing that makes rishonim so important is hard to tell. On one hand it looks like that entire age--especially in France was exceptional in logic. That is logical thinking. A book like most achromim write filled with fuzzy circular logic would have never gotten past first base. It would have been laughed out of court. There is something more however about the rishonim that is more than just logic.[rishonim might not be infallible but in terms of logic they always are]

But it is all I need to defend Musar. That is the idea that only the medieval writers had the capacity to  understand and describe in a logically consistent way the world view of the Torah.

Rishonim are medieval authorities. [Anyone from Hai Gaon and onward up until Rav Joseph Karo. ]

Achronim are people after Joseph Karo, including Joseph Karo. Rishonim also include the Gra.