If I remember rightly didn't the Planet Killer get destroyed during the gothic wars due to massive amounts of long range torpedo fire?
Also we're completely forgetting a rather large factor here, The Eldar, while they're not allied with the Imperium they're not going to sit around and allow chaos to take over the galaxy given that the more power Slaneesh has the more likely the Eldar are doomed.
The Eldar has better mobility than any other faction in 40k due to the webway, also the Chaos gods aren't all powerful even in the EoT this is evidenced by the ability of the Eldar to constantly show up and harvest spirit stones and leave before the Chaos Gods can do anything about it.
It's going to be a lot harder for Abaddon to push all the way to Terra with the Eldar constantly sniping and raiding his forces, another factor is the Harlequins but they would be doing much the same thing as the Eldar.
Also I don't think the Chaos gods can just shut down the warp to the people they don't like, if they could i'm pretty sure they would have done it far earlier than now, hell Slaneesh would have done it during the fall of the Eldar and wiped them all out.
The thing about Chaos gods is all the fluff points towards them not being true "gods" at all, merely extremely powerful beings that inhabit the warp, that's why they're so fickle about handing out power, they don't want anyone to have more than they do