In the end, it is the social value of your role what you get paid for, not your real productivity. Maybe not so much back in time, but as time passes and more efficiency is introduced in the economy by means of automation the less important is real productivity for your income.
Social value and value are not the same thing. Comparing teaching to farming makes zero sense and I think you might be misunderstanding the Baumol effect. It's talking about wages raising irrespective of productivity changes in the position because they are competing against other jobs with higher wages, as in competing for the worker.
It has nothing to do with social value, and social value has little to do with wages, unfortunately.
Because workers dont pick the low-paid shitty job when there is jobs they can do just as easily that pay higher.
Why work with low pay in retail when you can work in a plant on a production robot after a few days of training? The plant job most likely pays more, because the productivity is higher due to automation.
Retail can't become more productive because you can't automate it, yet you have to raise wages to make workers consider retail.