Interestingly, I saw more than one fantastic teacher get denied tenure in my graduate department (a top research university), and there was always a rumor float

Interestingly, I saw more than one fantastic teacher get denied tenure in my graduate department (a top research university), and there was always a rumor floating around that it was because they spent effort on teaching that could have instead gone to research. (One of them had written a textbook, and was told pretty explicitly that doing so had been a major mistake.) I've never heard of anyone getting denied tenure there or in any similar program for mediocre teaching evaluations. Meanwhile, at the private liberal arts college where I teach, most of my senior colleagues are just as committed to good teaching as my junior colleagues are. (The younger folks may be more innovative, but by and large they don't care more.) I've never seen someone's teaching go downhill when they get tenure; if anything, the research pressure eases up a bit at that point and they feel free to focus a bit more on the classroom. So maybe what you've observed about tenure is accurate in some places, but it's at odds with the places I've been.