That looks like nice work! I've experimented with using grosgrain instead of webbing, but haven't been pleased with the results when the straps are under load. It worked fine for my quilt under-straps, but backpack straps seemed to slip. What size grosgrain, and what size buckles? It looks like the grosgrain is bunching a bit in the buckles - does that increase the friction enough to keep them in place? I think you'll be happy with the durability of the ribbon - if it fails, it's easy to replace, as you found. I'd guess abrasion from the buckle will be the weak point, but at least that restricts the failure points to the longer strap (not the buckle attachment piece).
Plus, that orange looks uber-cool against the blue.
Do you have a way to compress the load from the top now? That's one benefit of the roll-top closure that you're losing here - I'd love to hear how it works for you.
And I totally agree about sewing in the foam on the shoulder straps - using spare socks just increases the fiddle-factor way too much for me. Objectively, any time I might "save" by being faster on the trail due to the .2 oz weight reduction would be more than eaten by having to stuff and un-stuff the shoulder straps every day.