Your claim with the api is outright wrong. The difference is between client side and service only api the rest is hyperbole. That gap is MASSIVE functionally. Client api is responsive, fast, access to local OS and local hardware. Service side has its place, but its service limited only. Severely limits access to other services which is very important when moving data.
You have conflated User Experience with User Interface. I didnt say UI for that obvious reason. The experience matters a lot. Having to open and process the same flow of task from one app to another app breaking concentration is bad fucking UX. Losing context is bad UX the list goes on. it reduces accuracy, & performance and ultimately productivity. Something as simple as classing becomes simple when the context of the conversation is very easy to get and more accurate when you dont duplicate an entire chain.