And if it is, what stops Google from just ... incorporating an LLM into their existing search offering instead of having their lunch eaten by this hypothetical IndexGPT? It's not like Google lacks expertise in LLMs.
You've also not addressed the more fundamental question of why an LLM would even be good for this.
You'd think that, but it's the classic innovator's dilemma. Google would have to modify their product to generate less revenue now to maintain market share. Except that market share will decline regardless as competitors rise, so they'd be cutting revenue just to slow the decline of market share. Alternately they could release a new AI product that cannibalizes their own search profits.
Either way, for a large multinational obsessed with quarterly profits and the stock price, it's very hard to overcome internal resistance to do either of those things.