Probably made other tweaks that gave marginal gains in ad revenue whilst neither caring nor measuring about the effect on search results.

Presumably ad revenue has gone up even as search result quality has gone down, proving that the ability to search your inbox is actually unimportant.

Unimportant, or unprofitable?

Computation costs probably not worth it for the incremental user satisfaction.

I'm assuming whatever they're currently doing involves machine learning so uses more compute than the simple correct solution that a CS freshman could code up.

Is probably less about making as revenue and more about not investing in it