I've used panniers on a rear rack for years, owned various sets (and various bikes), some good and some not so good, and it strikes me that... panniers are damned expensive. Sure, they're a good way to carry stuff, but they're expensive.
In an ideal world, racks on bikes would be designed to take a normal bag, the kind that we all own already, the kind that is a quarter the price of a pannier. Economies of scale dictate that a well made bag that is not a pannier is going to be far less expensive to produce (per bag) than one that can only be sold to cyclists.
Which got me wondering, what would a 'rack' for a normal bag actually look like? Might it be a basket above the rear wheel? Would that require a bike with wheels smaller than 700C, to give the required stability? And might it be worth investing in such a bike for riding 10 miles to work, and for local errands, rather than buying a set of good quality panniers?