One thing that happened much later, after Carmack himself left, was that id software stopped pushing the boundaries of engine development. Their last great innovation was magatextures, or virtual texture streaming in its more modern form.

Now cutting edge graphical features are mostly pushed by Epic and their Unreal engine. Like ray-traced global illumination, virtual shadow maps, virtual geometry, and fast ray-traced direct illumination.

But id software's games themselves arguably improved after Carmack left, despite not pushing technical boundaries. Doom 2016, Doom Eternal, and Doom TDA all were received very favorably at the time. Not sure whether this had anything to do with Carmack leaving though.