Sloped armor is no panacea. It would be more difficult for the overstressed German industrial machine to produce, and the Pz IV was by far the most important German tank design of WWII precisely because it was the only modern design Germany could produce in numbers. Further, it generally worsens ergonomics, and indeed the T-34 had some of the worst ergonomics of the war, which directly relates to combat performance.
An improvement in protection that would only be important later in the war, but fewer, less efficient tanks for the duration on is just as likely to hurt Germany as help.