There has been lots of discussion on this kind of issue over the years. Bottom line: it's a bogus diagnosis. Nobody expects all the stuff to go all the way to the street on a single flush. Do you bathe? Do you wash dishes? Do you do laundry? All these things put water in the line, and gradually move what is in there towards the street.
The sad part about this forum sometimes is that it highlights the immense differences in quality of licensed professionals out there. The guilds would like you to believe that all plumbers are knowledgeable professionals. The reality, based on so many "my plumber told me" posts on here, seems to be that there are a lot of licensed morons out there. The great thing is that this forum seems to lure wisdom out of the professionals at the pinnacle of their field, like Terry, HJ, Jimbo and others, for which I am so grateful.
Not to mention that what your plumber is proposing to do is illegal, at least in spirit.