“As far as this is concerned, I’m not sure how comfortable I am with the idea that learning about middos and self-improvement a) could ever be a bad thing or b) could ever be completely replaced by gemara learning, etc.”
I don’t think anyone would say that learning about middos and self-improvement is inherently a bad thing. But the way in which it is done, and the extent of the focus on it, could conceivably be detrimental. As for point b), I don’t know about COMPLETELY replaced, but it must be noted that a good portion of the content of Mussar Sefarim is Ma’amarei Chazal which discuss the mussar issues. For example, the Orchos Tzadikim in Sha’ar Haga’avah lists a whole slew of terrible things which ga’ava is compared to and some of its consequences. It is taken straight from the Gemara in the beginning of Masechet Sotah. So if you just learned the Gemara, you would still get the point that the Orchos Tzadikim is trying to convey. (Obviously, it’s not exactly the same, because you’ll be missing the Orchos Tzadikim’s interpolations and interpretations, but if you learn the Gemara with mefarshim, you’ll probably get a lot of that too.) An advantage of using the actual mussar sefer is that everything about the topic is collected together, but if you master Shas that shouldn’t be such an issue either. So the question would then be whether it would be advantageous to have a separate time, singularly devoted to the study of mussar concepts with intense focus on the ingraination of them into your unconscious via emonion and repetition (which was R’ Yisrael Salanter’s main chiddush), and engaging in activities designed to overcome your nature (more representative of the Novhardak school of mussar). The proponents of the Mussar Movement obviously felt it was, while those who were against it, such as R’ Chaim, felt that it wasn’t. But as previously mentioned, the Alter of Slabodka altered the style of mussar, and perhaps many of the objections have since fallen by the wayside.