There’s no lack of messenger bags, but they are all way too heavy and lack full closures and accessory pockets. The largest Timbuk2 bag is 3000ci! I use an REI messenger bag for daily commuting, but it is the same weight as my GoLite Trek (2 pounds and 4000ci) and about 2000ci.
Patagonia does have a good design for using silnylon or other UL fabrics and keeping the shape under control. The traditional messenger bag designs wouldn’t work well and the end goal is different. Messenger bags are made to hold documents and packages for short hauls. For hiking it’s more an issue of handling the volume of sleeping bag and clothing with a couple pockets for essentials and a water bottle, perhaps a big mesh pocket on the outboard side.
I forgot to mention that they are making an UL travel pack/day bag that is just 10oz. I’m surprised that they have utilized the UL materials but haven’t gone all the way to larger UL packs. 20L/1200ci, 10oz Patagonia travel pack
I think the whole haversack idea works for SUL and day trips only. Once you cross the 10-12 pound mark, they would get cumbersome.