Your very specific case without radiographs or a physical exam is laughable. Extracting primary teeth can cause complications down the road such as spacing issues and super eruption of permanent teeth. As the permanent tooth's root calcifies the primary tooth's root start to resorb and This allows the permant tooths tooth to grow to proper length by extracting the primary teeth early it can cause shorter roots in the permanent teeth.

I also have never understood why so many people question ever thing a dentist says. Sure there are some sketchy dentist out there but find a good on and trust him or her.