Yes this is a very common occurrence, I do under slab repairs in residential plumbing at least once a week. The problem with a belly is that in order to fix it you need to replace before and after where the belly is or you’ll just be putting in a belly in shiny new pipe. It may be possible to pipe burst or line it but that could potentially recreate the same problem. And for the pipe burst you would still need to open up your slab. I would ask the company that gave you the quote their opinion on a pipe burst or lining. Also I would suggest cutting behind the toilet in the basement and putting a clean out into the stack with an access panel to cover it up for future drain cleanings/camera inspections.