And no baggage. And you attract people who don't give a shit about KZ. Also KZ: SF was about light and snappy as you can get. No weight."Weight to the guns" is not a good justification for the prolonged existence of a franchise. Just create a universe that people actually care about and add weight to the guns.
I say "Just create a universe" as though it were an easy thing - and you know what? Creating a more compelling universe than Killzone would absolutely be an easy thing.