How much money are these companies going to make by reducing their weather app to 1MB, or 10MB? How much is it going to cost them to get there?

The world doesn't run on personal aesthetics, when nobody is willing to pay for them.

These days we have an AI system to do exactly this kind of optimization, too expensive for humans, and too cheap compared to consuming expensive modern hardware

The MacBook Neo has only 8GiB of RAM. This is widely decried as not enough. The RAMpocalypse means that 8 GiB is going to have to be enough or else it'll eat into Apple's margins. The Neos are wildly popular though because people have used it and 8 GiB is fine. So while the singular weather app being smaller itself isn't material, in aggregate, macOS being more efficient is something that lets Apple sell a lower spec product while still justifying a higher price and thus better margins for Apple.