Why do you think they will never be replaced by robots?
But I will say that until we have a robot that can fold laundry, we won't have a robot that can go into your crawlspace and replace a chunk of crusty old galvanized pipe with copper or pex.
Robots have excelled, so far, in controlled environments. Dealing with the chaos of plumbing in a building that has been "improved" by different people over the course of a century is the opposite of that.
The other thing is that the salary of the plumber or welder is in the range $20/hr to $40/hr. Can you make a general purpose, agile robot function at a total cost of ownership that's substantially lower than this?