According to IGN’s inside look of Infinite, the “rescue allies” and “liberate outpost” missions will be entirely optional. This quashes all concerns about Infinite being Halo Far Cry. feelsgood
Obviously though clearing outposts nets you huge perks like weapons, vehicles, variants, etc. So although it is likely 100% beatable without clearing any (or at least very few) outposts, the reality is that if you play on Legendary you are probably going to want to unlock some stuff.
Looks really good how 343 have approached this