I haven't used Reaper, but there are countless other examples in the software world where this works. There are many ways how this can be done. A good architecture, encapsulation, standards and handling of exceptions can deal with issues like crashes. Just because a script in a custom midi editor has a bug, it does not mean the whole DAW has to crash.
Then there is the other point I mentioned in the post: The marketplace. It is one thing if hobbyists make extensions vs companies that focus all their resources on a product. They will want to avoid that their product crashes because otherwise, no one will buy it. When you buy a third party extension you will also get the updates from the marketplace system.
So I don't see a problem why this shouldn't work.