Someone said on guild chat they actually don't spawn forever the other day. It doesn't look like anything spawns forever, even stuff like Sentinel affix on bosses seem to have a limit, I fought MODOK the other day when I was shit geared and I must have killed a dozen of sentinels but after that he stopped spawning them. Maybe the other affixes have no limits but I assume they do(just a lot higher obviously). I'm thinking sinister might spawn adds during his fight the entire time, but you'd need a way to kill the adds without killing him(and he teleports around and shit). Maybe a sweet spot outside the room where he aggros, spawn the adds then reset or some shit.