Nobody was learning!
Of course, since then things have gotten worse...Actually, I would question your suggestion that a teaching position was a super sweet patronage position or that teachers did not actually do anything. Especially when you consider that Americans were in general better educated when tenure was introduced than they are today. The purpose of tenure was to reduce the level to which teachers were accountable to the people who pay their salaries (the voters). This allows teachers to focus more on indoctrinating children into progressive ideals (which the parents may not approve of) than on actually teaching them unimportant things like how to read or do basic math.
The basic theory of progressivism is that the "elites" (in other words, the bureaucrats) know better than everyone else how things should be done. The progressive solution to government corruption is to reduce the accountability of government workers to the voters. Tenure for teachers grows out of that the theory that a government worker who is unaccountable to anyone will be less corrupt than one who has to answer to an elected politician.
Any program which runs right is obsolete.