that assumes that average and dim are constants and unchangeable. but the point remains is that we're not even getting the best teachers to begin with, because the job is so unattractive if it's not your passion.
Were teachers better in previous generations? I remember the majority of mine being excellent. Might it be more unattractive now than 30 - 40 years ago?
Inflation-adjusted average teachers salaries have fallen over the past decade, and starting salaries are at the lowest levels measured: https://www.nea.org/nea-today/all-news-articles/average-teac...
A lot of teachers in the US now work two jobs to make ends meet, which is a recipe for disaster.
Have inflation-adjusted salaries for other industries? They're not great either. Look at average attorney starting salaries.