re bracket supported bags etc .. I'd go for 2 types. Either pack light / small and use a good design for a stable and 'invisible' seat pack that you can still get behind if needed, just - score one there for Wildcat's seatpack.
Or, use reinforcements and mount it solidly so you can carry a bit more - but at that point I'd be just as likely to use a rack. Weight's lower down and I get better clearance for getting off the back of the saddle a bit. Could mount a light to it, guards if needed, etc.
In between bags ie a large, heavier seatpack, that may be the worst of both worlds ime although I've never had more than ~2.5kg in a Revelate style roll-up bag. Accepted that a rack isn't very on-trend right now but hey, function over fashion. New rack designs will be the NBT anyway.. if you're not carrying 20kg per rack they could go a bit lighter and more minimal in construction.
I'll be honest, I've never considered abrasion an issue.
- Worn through the saddle rail straps of 2 seat packs so far but the main body fabric of my kit lasts well enough. It'd be on my mind if I was riding a long tour with much off-road. Webbing with dry dirt in wears and starts to fray relatively fast I think. The rubberised Revelate straps are a benefit but they might not pull as tight in every area ie over a seat rail Wildcat-style, since they're stiffer.
The good thing about straps is that if they do break when your on a trip its easy to bodge them up whereas broken brackets, especially alloy ones, are much harder to bodge.
Agreed .. I ride in shorts that have a webbing belt holding them up, could keep a bag attached and just let my pants fall down here and there : )