The assertion that this is because "indexing the whole Web is crushingly expensive, and getting more so every day" is a bit flawed. Since old content is very unlikely to be updated, it doesn't have to be re-crawled a lot. I'm certain Google has a score that tells it how often the content of a given site is likely to change. This argument of expense becomes even less durable when you consider that DuckDuckGo, a company with an infinitesimal fraction of Google's resources, is perfectly able to keep that kind of content in its database.

I agree with the observation that this is about shifting everything to current data, because people overwhelmingly care about things that happened a few days ago. There used to be a long tail of users searching for old data and references, but I suspect they're fading away. Biasing the index towards recency also has legal advantages for Google, because delisting old content makes it less likely to receive takedown requests in connection with "right to be forgotten" legislation.