The general consensus is that Alien: Isolation is a pretty solid game. At the very least, it's leagues better than the previous two major Alien titles: Aliens vs. Predator by Rebellion and Aliens: Colonial Marines by Gearbox Software. For that reason, I doubt anyone would be too shaken up by the fact that neither AvP nor Colonial Marines are available on Steam anymore.
Games are delisted from Steam all the time, typically when its license agreement runs out (see also Marvel vs. Capcom and Scott Pilgrim). Then again, Colonial Marines is barely two years old, yet it got yanked at the same time as one originally released in 2010. These two as well as Isolation are all published by Sega, so this could simply be a case of Sega removing titles that have been critically maligned so that only the cream remains.
Both could feasibly return at some point, but for now, if you want to exose yourself to AvP or Colonial Marines, you'll need to track down a used console version.
