You CAN host your own Tangled AppView. You CAN host your own knot. You DO NOT have to in order to self host your own PDS. Each of these layers are decentralized from the other.
The appeal of bobbin is it's fully stateless and pretty lightweight. IIRC you can run all the infra to run a proper tangled appview (including bobbin) all on your average lower middle end general purpose cloud instance.
Bluesky’s decentralisation is a “yes but it’s complicated and you can’t _just_ do anything”. I like that they’re experimenting with “apps” but source control feels a bit too far.
There are lots of organizations out there, even multi-billion-dollar ones, using Google Workspaces.