But for most professions the change will be that individual workers will become more and more productive. In those cases it doesn't matter if the individual is a checkout assistant, airline pilot, hooker, journalist, road-digger, marketing person or doctor. Technology, AI, new processes, more disintermediation, will mean that a piece of work that requires a given number of person*days now, will require many fewer as time goes on.
That leads to two possibilities: either professions (and manual, low-skilled work) will need only a fraction of the number of employees that each sector has today. Or that more opportunities will arise for the same number of people to be gainfully employed - but do we really need more holes dug in the road, or more blogs/newspapers/magazines in our lives?
Clearly, this won't happen all at once. But as people retire or leave their traditional jobs, they won't necessarily be replaced. Maybe new types of job will appear (we didn't need programmers until the computer was developed) or maybe we will see an underclass of unemployable people emerge, supported by a UBI system that is paid for by an ever-shrinking number of salaried staff.