Tradespeople are (economically speaking) substitute goods for buying a new good factory made. You can hire a guy to repair your washer or buy a new one.
In the future it will be even cheaper to buy new so there will be less need for repairing or custom work. As for the rest - well computers can drive, why shouldn't they be able to help your wiring your house? Or why shouldn't a single electrician (probably in India) be able to help 10 non-electricians with HoloLens wire a house?
I agree, some trades people will design solutions to problems, some people will just turn up and screw in pre-assembled units. The existing category will split into two.
