So untrue and unrealistic. You really have no knowledge of economics. Lets take the example that you were responding to of the nanny who was making $21 an hour because she had been in her position for 5 years.
With annual $1 raises and no salary raises for additional children this nanny started $16 an hour. Its more likely that a nanny who has been with a family for 5 years has seen additional children which would have been a $1-$2 salary bump. This situation with annual $1 raises puts her starting salary $14-$15 an hour.
A nanny who was competitive at $14-$15 an hour 5 years ago is NOT competitive at $21 an hour now. The market has decreased or stayed flat in the past 5 years and there certainly hasn't been a 30% increase in nanny salaries. Five years of experience is not a significant enough distinguisher to make a difference between $14-$16 and $21. She may have finished a degree but even this would not jump her $7 an hour over her previous starting salary. A nanny who doesn't drop her rates for a new job has either a

had short term jobs and/or no pay increases w/ a longer term job or b

hit the jackpot out of sheer luck.