This is often said as a “gotcha”. I appreciate you don’t seem to mean it like that, but more as a genuine question.
On a societal and economic level, the idea is that people should not be in lower paid jobs for life. They should make progression through them. (This obviously varies on an individual level according to a persons needs, wants, and abilities).
So, for example, someone starts on the tills in a supermarket as a school leaver, progresses to supervisor, to deputy store manager, uses the transferable skills to become area manager for a company in a different industry, and so on. In the meantime, obviously someone else has started on the tills.
There is never a scenario where the lower paid jobs stop existing, but they are (many of them, not all) supposed to be a rung on the ladder, not the only step.
Of course some people can’t progress, some don’t want to progress, some make an active choice to stay for various reasons. But the argument that society will collapse if people have aspirations is highly flawed.
Anecdotally, I know someone working as a care worker while waiting for her nursing training to start. Someone working in a nursery while training as a teacher. Someone working as a cleaner while building her business. There is nothing at all wrong with the jobs they are doing now, and they are proud and happy to be doing them, but they don’t intend to be doing them in 5 years time. And, more relevantly, in 5 years time when they have moved on there will be someone else filling their shoes.