I have cows that are getting old and I will never keep a heifer from, they're here because they raise a good calf every year and they're no trouble, but they would be the ones I'd cull on age.
Then I have cows that produce outstanding calves with any bull they're mated to, as well as having all the other good sides of a productive cow.. I'll keep them as long as they work for me.
This turning over the generations as quickly as possible sounds like it's done to chase a fad... I'll just take birthweights as an example.. Less is good, so even less is even better, right? So lets breed cattle with 60 lb birthweights.. we'll never have a problem calving, they can come out sideways if they need to. However, if attention isn't paid, you're going to get cows with less and less pelvis and you can be setting yourself (or customers) up for disasters when the cow is required to deliver a 100 lb calf.. something that isn't extreme or uncommon.
I look at losses at birth as a bit of a test... if you never have any, there must be some place you're losing out on.. if you have too many, you've got something wrong in the program.
Same goes for milking ability/breed back rates.. If all your cows breed back every time, that is good, but perhaps they could stand to put more energy into milk at that point.. if too many aren't breeding back, perhaps they're milking too much for the nutrition available.