So Legends just flipped everything upside down, but damnit we're still stuck with the Hawk twins for the time being.
In their season finale we find out not long after Savage conquers the world an alien race attacks and only through Savage do we defeat them. Along with that the second boot drops and we learn why "Time wants to happen" as the time council has a magic device that allows them to modify time and everything Rip has done was engineered to put Savage in power. Learning this of course our heros blow it up, with a noble sacrifice required which the Atom takes, only to be knocked out by Mick, who is then knocked out by Snart. Thus 'dying' in an explosion of something able to modify the time stream is his exit as a show regular.
Its interesting that the arrowverse earth eventually faces alien invasion and an alien has recently made friends with the flash and is also now on the same network...