From what we know it seems a little early in Destiny's supposed 10 year plan to kill off the emperor if the Cabal empire is half as expansive as they want us to think it is. Especially because the Cabal don't have a ton of fleshing out yet, I think they're going in another direction. Supposedly, with the exception of the Skyburners, the Cabal units we encounter are just scouting units. I think it's just going to be the Cabal finally mobilizing an actual invasion fleet, not just a small force holding their Martian toehold.