They don't have to do that today, though. Remember original Google? Or original Gmail? Neither had prominent ads for the first few years, a couple decades later the ads are everywhere.
OpenAI isn't burning through tens of billions of dollars every year on its free tier for charity. It will dial up the advertising knob (and every other knob) to 11 the moment the cash starts to runs out.
The more they emulate ChatGPT's clean UI, the more they are failing to push ads in people's faces, which is what generates that $100B for them.
Their business model fails if their users don't experience a confusing crap-fest of ads.