Google's main product is ads, not search. Search (and Mail, and Video) are just mechanisms for offering ads.

By balkanising the internet they can have a monopoly over offering ads to certain target groups that fall within their walled garden.

That isn't how this works, Google doesn't have good enough content to compete with other tech giants without the open web. If Google kills the web then Google kills itself. Most of their money comes from their search web page and not the ads from other sites, so people browsing the web is their main source of revenue. Their other sites are big, yes, but not big enough to make them one of the few most valuable companies in the world.

Anyway, Google works hard to make the open web competitive with all the walled gardens around. When they do that they put up some fences to make the experience a bit more like walled gardens, but the open web is still just a few seconds and a click away.

> If Google kills the web then Google kills itself

That's why Google instead reshapes the web as Google sees fit.

> Anyway, Google works hard to make the open web competitive with all the walled gardens around

No. It subsumes the web and replaces it with all things Google. It dominates all web-related standards bodies, pushes through half-baked Chrome-only APIs as "standards" and then presents them as fait accompli through Google-controlled properties like web.dev.