This is untrue.
Halo's difficulty came from its unparalleled artificial intelligence, there is no way in fuck that a health regen system is going to save you from a Covenant Zealot if it's coming for you. I far prefer Bungie's method to a system where the developer is incapable of making intelligent AI (Valve) and so instead to rely on fake difficulty through skimping on health packs instead.
Talking about MP.
How is that Halo 2's fault?
It's success inspired copycatting in the same way COD does now?