There's a few ways to do that. The easiest is to point out that almost any lucrative search query has -0- organic results above the page fold on a typical monitor today. It's all ads unless you scroll down.
Then, it's not proof, but how much time do you think Google spends on things that sit below the fold and aren't clicked on much? What would the financial incentive be?
Quality organic search results is the reason they can have ads above the fold. There's tremendous financial incentive for them to care about that.
How would you demonstrate that being 50 years old is worse than being 25 years old?
You ask people that are 50 years old or older because they have been on both sides.
> it's a result of Google's algorithms
Well, it's simply in front of your eyes: this [1] was not possible in 2006.
Anyway, the fact that you cannot easily find on Google why Google search results are worse, proves that Google search results are worse today than in the past.
https://www.theverge.com/tldr/2020/1/23/21078343/google-ad-d...
How would you demonstrate that search is objectively worse? And how would you then show that it's a result of Google's algorithms, and not a consequence of the content of the Internet changing significantly?