So... I don't think this is certain. A surprising number of people pay for the ChatGPT app and/or competitors. It's be a >$10bn business already. Could maybe be a >$100bn business long term.
Meanwhile... making money from online ads isn't trivial. When the advertising model works well (eg search/adwords), it is a money faucet. But... it can be very hard to get that money faucet going. No guarantees that Google discover a meaningful business model here... and the innovators' dilema is strong.
Also, Google don't have a great history of getting new businesses up and running regardless of tech chops and timing. Google were pioneers to cloud computing... but amazon and MSFT built better businesses.
At this point, everyone is assuming AI will resolve to a "winner-take-most" game that is all about network effect, scale, barriers to entry and such. Maybe it isn't. Or... maybe LLMs themselves are commodities like ISPs.
The actual business models, at this point, aren't even known.
I don't understand why people believe this: by settling on "unstructured chat" as the API, it means the switching costs are essentially zero. The models may give different results, but as far a plugging a different one in to your app, it's frictionless. I can switch everything to DeepSeek this afternoon.