Wondering about worms in dogs and how to get rid of them once and for all!?

We got our dog (as a rescue) two months ago. He has been to the vet and we have de- wormed him twice ( on the 20th of July and August) with inceptor.

Today is September tenth and I found a 4 inch dead round worm in his poop.

I took a fecal sample to the vet and he confirmed it was a roundworm and said not to worry- to continue treating him on the twentieth of every month until it snows.

My question is... If he obviously has worms- is only treating him once a month allowing worms to grow and lay eggs and continue an everlasting cycle? I hear they can hatch and lay hundreds of eggs every 8 days. Meaning there will always be hundreds of eggs that never get killed...

I should stop googling and listen to the vet.