Man, if it was that simple, ALARM wouldn't exist and arch would have (at least) allowed aarch64 packages in the AUR years ago. There are other factors in play than what I listed above, but it's still weird to not even mention the existing community efforts.
Reproducible builds are one step closer than what ALARM was already providing. It's cool, but it's still just one step up, and a rewrite. I don't understand why Collabora went in this direction aside from having more billable hours.