I happen to have C&C, but I agree that a "free" old school ruleset would be a nice option. Also not criticizing vodokar's excellent work - it makes a lot of sense to build off of C&C. But in the era of the 99-cent app, 10 USD will prevent some people from using it.
It's definitely possible to make a ruleset that works for multiple OSR systems, but my guess is that it wouldn't run any of them as well as a dedicated ruleset - especially AD&D with its rules complexity. C&C is also tough to lump in there with ascending AC, although vodokar was able to figure out how to flip it around for AD&D. Either way, making a ruleset is a ton of work, and people will always want it to do something it can't :)