I have no doubt it's possible to do most things with just one big bag, and one small one. Having said that, I have two Kifaru frames and a slightly embarrassing number of bags, as well as one internal frame bag ... my gear room is a little like being able to walk into their showroom, and pulling down exactly what I want.
This might be excessive, but it has some benefits - I get exactly the right mix of waterproofness, internal vs external carry, fixed vs movable pockets, single entry and internal bag vs multiple, and so on.
While this has been a pretty expensive way to go in some ways, I've been judicious about picking up the bags on special, or secondhand. And, as it was the first platform I went with (except for old hiking packs from the mid-90s, still going strong), I didn't have to 'bridge up', and lose money when selling on each rung of the quality layer ... all the money I might have lost that way basically went into having as much flexibility as I could want, and then some.