So when the new Taniks strike was coming out I saw quite a few posts asking why SIVA is still around if we blew up the replication chamber. SIVA is able to self-replicate, so while the giant mass production device is gone, the SIVA that's still out in the world can continually replicate and spread. There is no way to actually destroy SIVA completely, short of vaporizing every planet it's had contact with. The new dialogue from the Shadow Thief strike also explains that Taniks was already a splicer (not with SIVA, but just normal technology) and was possibly a lot more machine than anything else. That means that all the SIVA splicers had to do was go to the moon and repair the damage from when we killed him the first time, just like Sepiks Prime. The Nexus is a bit less logical though. There are still Taken forces in the universe, and I guess they've been trying to amass their armies in secret while we've been fighting the SIVA splicers. Now it's not really explained (at least I didn't catch) why they went after the Nexus mind, but my theory is that they wanted to link into the rest of the Vex network in order to take them all. The fact that this strike starts out with Cayde being secretive about the goal is kind of pointless. Like why not just say "Hey, I think the Taken are trying to take over one of the Vex's most important links. Stop them."?