Replaced the brake pads and discs on my '05 Santa Fe

I noticed a squeaking sound whenever I stepped on the brakes, so I felt something was wrong and went to the repair shop.
They replaced the front and rear brake pads as well as one disc.
I didn't know much about the parts, but when I compared them to the new ones, I realized the wear was massive.
They said it could have led to a serious accident; is that true...?
Since the car is quite old, they mentioned that if they couldn't find the right parts, they might have to custom-make them.