It's possible, so I looked the research question up. Looks like my instincts were right.
https://www.sciencedirect.com/science/article/pii/S0742051X20313810 The study is Australian but it cites other studies too for its findings.
"Again though, this evidence is not clear cut, suggesting that there is some effect for experience but that this is limited and not cumulative. For example, some research across national and international contexts provides evidence of an early effect for experience whereby beginning teachers quickly improve, but this association declines after their initial adjustment to the field."
Basically the research seems to indicate what I was trying to say. That there is not a linear progression in years of service and performance. There is initial improvement, then it plateaus. If this is so then why the pay raises beyond cola for older teachers? No real reason. It is just a reward for staying on the job. Perhaps it is good for retention but the low starting salaries for newer teachers hurts on that end. I will note the decline is far faster than I expected. Burn out happens though.
- Teaching quality observed for 80 teachers with 0–3, 4–5, and >5 years’ experience.
- No evidence of less competence for teachers with 0–3 years’ experience.
- Some evidence of decline in teaching quality in teachers with 4–5 years’ experience.