Electron is somewhat different than a PWA. They are not the same.
Electron packages up its own browser and something akin to a node process along with your code. There is effectively no sandbox on Electron. Last I played with it the minimum bundle size is ~80MB, depending on the platform and feature set and it runs somewhat larger on disk after install.
A PWA uses the existing browser on your device and must abide by the browser's sandbox and limitations. It's basically running as a page tab but skinned to look like a native application.
For both Slack and Spotify, I would prefer a PWA over electron. They work fine within the sandbox and then it's simply smaller/faster and your browser, the one major dependency, updates separately. That's less bandwidth to keep everything up to date, less resources both on disk and in ram. Given you advocate using the browser for Spotify, it seems that you would prefer it to be a PWA as well.
There are pros and cons to both approaches, but from a resource perspective, PWAs are potentially quite lean. The last PWA I published has a download footprint of about ~800k, uses ~3MB on disk, and ~30-50MB of ram when running, and ~1s load time on my phone. It's pretty much a big ball of javascript, light on pics or media other than the icon set - but runs an inventory system and allows some modest camera assist from the device. Contrast that to a React-Native application I produced of the same complexity at a previous employer, it was 30MB download, 50MB on disk, and even more ram, and 3 s load time on my phone. PWAs potentially open faster than similarly complex native app if the browser is already running and in memory - which is a common case.