Then the competent of those 500 plumbers start their own plumbing business and do very well competing against the rest.
except they dont have money for materials / branding / customer loyalty / initial wages. The bigger company can easily drop their prices to force you out of the market.
Of course with enough determination anyone can create a business but lets not pretend someone has stumbled upon some new economic principles. Its supply/demand.
The only way to encourage people to do manual labour in a truly fair society where jobs are few is for them to do it for their community / respect.
I think a good system would be to assign people who live in an area to manage that area's plumbing without redtape.
As a simple example:
Job A - Cleaning toilets. Duties - clean the toilets once a day using supplies from the closet - if anything goes wrong report it to the council / building manager.
Job B - Toilet manager - Duties - spent annual budge of $5000 on supplies and setup a storage area. If there is leak/problem learn to fix it yourself or hire a plumber. Analyse daily usage of toilet. Propose toilet expansion and alternative revenue systems (50c per use). Do market research to see if the people in the area are happy with these toilets. Fix chipped tiling. Report vandalism / drug usage.
Empowering employees has always shown to increase job satisfaction.