For me personally, it's having a set of UI widgets as consistent as possible with everything else on the platform as the primary concern. Secondarily it's performance / battery life.
People don't expect Google to look 'like Windows' just because they're on a Windows OS, they expect Google to look like Material apps across all platforms. There's not 'windows spotify' or 'android spotify' but they expect the app to look the feel the same everywhere, and in that way companies actually benefit from using Electron.
I don't know what Slack or Teams would look like with native UI components, but I am pretty sure I would consider it inferior to what we have at the moment. The possibly improved performance and memory footprint would be nice but not enough for me to care about.