> Wouldn’t it be nice if major services supported fully-featured apps inside their client?

This is everywhere in China: WeChat, Alipay, etc. all have mini apps running inside them.

Honorable mention: webxdc, as implemented in DeltaChat for example.

https://webxdc.org/

It’s also a good way to increase the attack surface of a service. The more connecting points/components you have, the harder it is to secure, and the easier it is to hack.