The key thing to understand about Google (the search product) is that it does search somewhat well but it is extremely bad at synthesis. When it comes to big data, synthesis is just as important as search. The UX with traditional 2000-era search engines involves the user being given a library's worth of information rabbit holes to dig through. With synthesis engines, the UX is completely different and they might even be solving different user problems.

As per your example, nobody currently uses Google as a way to draft letters, but rather as a way to learn how to draft letters. I think the distinction is pretty key in understanding the difference between the two problem spaces. I would think that "write me a letter" is a problem that isn't in Google's domain. I do not think synthesis engines will necessarily replace search engines, but the two will both be useful.

The premise of Google's interaction design is that you will be taken to an external resource. Google in recent years has started adding widgets and blurbs at the top of the search results for common things like stocks, covid-cases chart, weather, etc. but this synthesized content isn't their primary focus and are likely hard-coded to a large extent.