The problem would be cross-platform support. In the end the capsule file is the standalone binary similar to a PDF or a Word document, you only need the viewer app to open any capsule file.

you can trivially bake X binaries if you have the viewer app for each platform. This is the normal way to distribute software for cross-platform support.

I might just be hyper-defensive to any sort of vendor lock-in like a "viewer app".