If you take away FTL travel, you trip over travel times in the original movies that depend on FTL travel. Unless we're supposed to think FTL travel was invented between 2120 and 2122?
I'm not saying the logic and physics of the Alien universe is watertight. I always assumed they have near FTL travel, i.e. maybe a year to Alpha Centauri ( so roughly 25% of lightspeed) with their tech, which would explain the years and decades they are underway and the cryopods.