Bikepacking - a viable alternative to racks & panniers

Not really - bikepacking style bags are all long and thin. The Carradice is wide and deep(ish).

Bikepacking saddlebags were designed to hold predominantly soft items that can be cinched down really tight, they also require a minimum amount of said contents in order to function properly and not swing like crazy. They weren't designed for carrying random hard objects.
This is why I see so many people riding around with bikepacking saddlebags flopping and swinging about behind their saddles,with hardly anything in them. They've become fashionable, but for a lot of people aren't the best choice.
A full frame bag would make more sense if you really have to carry it on the bike, but even then it will need to be well padded.