Correct. XPlane scenery is not streamed, but pre-downloaded.
The way the gateway system would differ in MSFS is, after the community submissions are approved by the devs, they integrate it into the 2PB of scenery that is streamed from their servers, so every user is able to view this new handcrafted POI for example without having to download any further addons as this has been added in the base scenery of the sim, which is the whole idea of a Gateway system.
You raise valid points. While we can’t speak for the costs or the legalities of this system, they can implement certain strict terms and conditions such as no Google Earth ripoffs, no excess polygon counts, can’t use licensed textures etc.
If you’d like, you can view my topic ‘Scenery Gateway System’ on the Wishlist category which explains my proposed MSFS-compatible gateway system in detail.