That is just dumb to assume there is a replacement for low skilled labor, when no replacement may exist.
In the case of janitors, Roomba and self sanitizing surfaces have been making strides, but not enough to replace the necessity of their low skilled labor.
Although some do not.
Arbitrarily increasing wages to eliminate low skill work won't necessarily make people more skillful, some people just don't have the motivation.
That's why you sometimes find middle age people working at McD's.
Landscaping is just an easier field to enter, because the skill set is easier to learn.
If there is a learning break through making engineering just as easy to learn, the relative value of engineers with eventually come on par with that of landscapers.