Hi there, I work on Fuchsia, specifically on our Software Delivery system [1].
You hit on exactly the right point: it's _possible_ to download and run software on demand, but it's also possible (and recommended) for products to turn off that capability if it's not useful or valuable for their use case. We pin packages for the base system itself, as well as lots of configurations of products.
The ability to run code on demand is really valuable for our development flows and quick prototyping: built a new test or experiment? No need to update your device, just try to run it, and it runs!