You mean, apart from things like writing parts of the Windows operating system, or JVM's, or Compilers, or PC Emulators, etc...?
I know very well - from first-hand experience over many, many years - how much better it *would have been* to build in modding ability; but things have changed.If you don't build in this capability from the start it's bloody nightmarish to implement later, much better to build the entire gameplay aspect from the same interface that the modders will use, that gives you the most flexibility later on, and the least effort refactoring.
Given the codebase as Josh has described it, if he releases the source, and if someone builds anything on top of it, it would be a much bigger effort to fix and complete the Lua/C interface and write the game according to the last plan Josh espoused, than would be reasonable. As Josh said, it would be years more effort.
Apart from that, don't forget that a modder would have ultimate access to the source code going forward, and wouldn't need a limited, scripting language interface to mod the game anyway, and if you even did all these things, there's no telling if it would actually perform well enough to play as a game.
My approach as I've listed, is borne solely from the need to strip down the engine to the minimum viable product; and to build a game from there in the shortest, easiest, most distributed way possible. It means cutting out a great number of features, and the ability to mod any subsequent game via a scripting language is very high on my list of things that should die given the highly limited functionality they would bring.
Putting it simply, would you rather have a game you could play, or a game engine that you could mod?
I totally accept that your priorities for a game are different to mine, and modding may be the one thing you wanted, but I think we can definitively state that you're not going to get that now, so from here on, you might want to alter your expectations.