"In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work." -17 U.S. Code § 102 (B)

Under US Copyright law it is entirely legal to make an open clone of Games Workshop's rules.

If people are tired of GW bullshit about "Counts As" and retiring old units and stuff then JUST FORK THE RULES and start playing that instead, with community iterations of the point values. Hell, there could even be points values for Warmahordes and Star Wars Legion.

You don't need a company to keep a game going. SWCCG tournaments lived for decades after Decipher abandoned it. What you need is just a critical mass of people who play it to give the rule set enough momentum to be worth having tournaments. If an open, community managed wargame caught on, it would take the wind out of GW, who have been extremely poor stewards of their game.