I've already mentioned ChromeOS as one counter-example.
SerenityOS and Ladybird browser forked but until recently had a lot of overlap.
LG's WebOS is used on a range of devices, derived from the Palm Pre WebOS released in 2009.
The gigantic special OS is baggage which already has been cut loose numberous times. Yes you can run some fine light Linux OS'es in 4GB but man, having done the desktop install for gnome or kde, they are not small at all, even if their runtime is ok. And most users will then go open a web browser anyways. It's unclear to me why people clutch to the legacy native app world, why this other not-connected mode of computing has such persistent adherency to it. The web ran a fine mobile OS in 2009; Palm Pre rocked. It could today.