A lot of theory in the OP that does not, based on my experiences, translate into reality.
I started out using Beckman Cordura bags that I am not sure are even coated. They have rain covers and are highly compartmentalized inside and out, although some of the internal dividers unzip to allow you make larger pockets. I now use Ortlieb Sport and Back Packers Plus models.
First, unless you inadvertently leave the lids open, how much water is going to get inside the bags if you need to retrieve something while it's raining? Your body alone will shield the rain while standing over them. During my last two tours I had to break out the foul weather gear during several rain events, including at least one heavy downpour. My other gear inside the bags did not get noticeably wet.
I, too, have had zero problems with moisture in the Ortliebs save for one early use when I did not properly secure the lid of the small, outside pocket of one of the Back Rollers. Some rain water got inside. There was maybe a half inch at the bottom. When I notice it at the end of the day, simply dumped it out.
I have noticed no difference in drying of damp items inside the two styles of bags. Condensation on anything that goes inside the bag is a rarity for me, not the norm. I also keep my tent out of the bags. I carry one plastic shopping bag in the event I need to put wet clothes in one of the bags. Another option if the day is dry is to lash west clothes to one of the racks. Air flow created while riding is an excellent way to dry things. Any sun helps. One handy feature of the Beckman bags are the mesh pockets on the outsides of the front bags. Great for drying damp clothes while riding.
A for having mainly big compartments (Otelieb Packers do have some small, segregated spaces inside, and the Back Packers have small, external pockets that come standard. You can also purchase additional, external pockets.), I first thought I would have a tough time adjusting to the Ortliebs. I did not. There are very few items I need access to while in transit. The overwhelming majority of the time it's rain gear and/or my wallet. I put these items where they are easily accessible. Even in camp I never find myself wondering where I stashed this or that. I organize by bag, with the stuff I will likely not need while in transit (e.g., cooking stuff, off-bike clothing) going in the right bags.
Overall, I am happy for the waterproofness of the Ortliebs. Messing with rain covers was sort of a pain at times. I still have my Beckman bags and, for various reasons, want to use them again. Unfortunately, someone stole their companion custom racks, so they sit idle in the basement. Hoping this is the winter I have the bags and my current racks modified so I can use the bags once again.