Witchers were created specifically to fight monsters and as i understand it, killing monsters require a lot of knowledge and preparation.

So how is it that witchers are no longer needed by the time of witcher 3?

The conversation with the captain of the guard and the duchess in B&W seems to imply that most people have next to zero knowledge of monsters.

If there are fewer monsters in the world would'nt that be because witchers have been doing their job well? Once all the witchers are dead and no new ones are created, wouldnt the number of monsters increase?

I have not read any of the books. Just played the games. Any clarification would be appreciated.
Thank you.

If you read the books, at the begin, was a lot of monsters and few of humans, now(when our hero live), it is inverse, a lot of people and few monsters, so, in the time begin(book speak of it as Collision of spheres), when 2 people of 10 was murdered , was a big problem, now when 2 from 10.000 are murdered , it is more a annoying problem, most contracts are put by close relatives/friends/people of victims, than the authorities(even when they put a contract, it is more because of losing of money not of life)...

Geralt says they're not as needed as before, but the world of TW3 makes it seem that monsters are still several times more numerous than humans in any given region. Toussaint is supposed to be a relatively untouched, orderly state, yet it's still replete with monsters everywhere. The perils of open-world games, I guess.

As for the lore, when the Conjunction of Spheres happened, monsters appeared in the world all at once, that's why there were so many of them. Now that some time has passed, their populations have been culled. Yet a good number of species can still sustain their numbers though reproduction if left in peace.