I know you're not a "teacher" per se, but isn't it a bit hypocritical that you should speak of tenure systems for teachers when, lets be honest here, tenure for university professors is pretty much a license to be a shitty instructor. Now I know you're thinking "teaching isn't the main aspect of my job", and that may be true for you and those at your university, but not every school is a research university. It just smacks a bit of a double standard.
No I get that.
My response would be along the following lines.
1. University tenure shouldn't be a solid as it is currently. Tenured faculty generally go through 5 year reviews after tenure. But there should be more pressure to keep performing to get raises, and more ability to get rid of people who aren't doing anything. It's not just an issue with teaching, there's issues with people who greatly reduce (or stop) publishing research, getting grants etc. after tenure or after becoming full professor etc. as well.
2. We need more split between research and teaching in research universities. It's already there in the sense that research faculty like me typically have a 2-2 teaching load (two course in fall, two in spring) where as full time teaching faculty are on a 4-4 or 5-5. But most of the full time teaching faculty at research universities are not tenure track faculty unfortunately.
That should change, and teaching faculty should be evaluated on teaching only, research faculty mostly on research as currently. Totally on research if we get to some perfect world where top researchers aren't forced to teach at all.
One thing our department is trying to pursue is variable teaching loads. So some people who are good teachers but don't do much research can get say 3-3 loads, and the more research active faculty can get 2-1 or 1-1 loads, and buy down to even lower loads from buying out classes with research grant money.
3. For teaching universities, tenure evaluations would be totally based on teaching (just like for the teaching faculty at research schools). And evaluations should continue post tenure to make sure they're still doing a good job, just like research faculty should keep being evaluated on the teaching and research they do post tenure.
But all that said, "teaching" shouldn't matter as much in higher education if our k-12 system wasn't broken. College used to be (and should be) a place for the best and brightest scholars who by the time they get to that level don't need a lot of direct teaching. But rather can just be lectured to, asked to think critically about material, and most of the learning being guided self learning outside of the classroom. Outside of very advanced classes in math and the hard sciences of course where more teaching is needed.
But our public education system is broken, so only a minority of college students are ready for that type of guided self learning even in simple classes like criminology. Anything I don't teach in great detail in my lectures, the classes tend to do poorly on those areas on exams. I end up having to lecture a lot on stuff that's covered in the readings in details as when I just expect them to read and understand material on their own, and use lecture to cover additional or more advanced material, they end up bombing on the book-only material and doing better on the class-covered content.
So in short, college education has really gotten dumbed down to pretty much the high school level--with just more detail given to specific major area subjects.