Working on pipes running through walls will never be easy, and someone who does it for a living will always be quicker at it than you, and is far less likely to keep the drain runs at the correct slope, pipes away from areas that get too cold, and install the access points for easy cleaning.
These are ALL things I've had to work around fixing plumbing in houses I've owned where various bits of plumbing were done by amateurs, and I'm sure I am busy leaving behind similar issues for future owners...
The time thing can't be over-rated either, a plumber may charge 80-200/hour, but they get a lot more done in the hour, having all the various tools etc. (and yes, you will always need tools to deal with pipes that are 50-100 years old, the modular snapping will lock up, two layers of remodeling will block key joints, with the right too getting the reach needed, etc.).
Plumbing is here to stay, doesn't mean we won't get a glut in plumbers though.