It's not that wild of a conspiracy theory. Hard to get definite proof though because you would have to compare actual search results from the past with the results of the same search from today, and we unfortunately can't travel back in time.
But there are indicators for your theory to be true:
- It's evident that in UI design the top area of the screen is the most valuable. AI results are always shown there. So we know that selling AI is of utmost importance to Google.
- The Google search algorithm was altered quite often over the years, these "rollouts" are publicly available information, and a lot of people have written about the changes as soon as they happened.
- Page ranking fueled a whole industry which was called SEO (Search Engine Optimization). A lot of effort went into understanding how google ranks its results. This was of course done with a different goal in mind but the conclusions from this field can be used to determine if and how search results got worse over time
- It's an established fact that companies benefit from users never leaving the company's ecosystem. Google as an example tried to prevent a clickthrough to the actual websites in the past, with technologies like AMP or by displaying snippets.
- If users rely on the AI output Google can effectively achieve this: the user is not leaving the page and Google has full control over what content the user sees.
Now, all of the points listed above can be proven. If you put all of that together it seems at least highly likely that your "conspiracy theory" is in fact true.
