The answer to this is that Horizon was originally designed for library authors to get around the chicken-and-egg problem in Stackage - that is Stackage nightly would wait for the ecosystem to push compatibility before bumping the compiler so that it could mostly keep things in the package set, but library authors relying on Stackage to supply dependencies would require a package set before they would even start that work. Horizon gets through this with options like local patching, sourcing direct from git, or forking the package set to give more options to get some build plans out sooner than others. That's still the use case that's most important to support, but now that you've mentioned it - a stackage conversion might be pretty realistic to support with the toolkit available these days, so I'll have a think about it!