By "stuck in the ruts I meant keeping the design the same for some interval of years - in the bags you should be able to make updates more often than in the bike since no replacement parts are involved and if there are they can be made the same. At the more general level, I am myself in favor of gradual well thought through changes rather than frequent erratic and possibly misfiring.
If we look i.e. at the S-Bag: It was invented in 2005 along with the S-Bars as the existing Bags would be too high. About two or three years later it got an overhaul, small changes on the bag design, an yellow inner liner (instead the original black) for easier spotting of bits and pieces in the bag, a via velcro exchangable flap (to change the design whenever you like) along with a couple of different flap versions and the rain cover became standard. At the same time the C-Bag was introduced, very much inspired from the experiences with and design changes on it's smaller sibling. The bag wasn't changed until 2015, just a bunch of different flaps came and went. In 2015 or 2016, both, S-bag and C-Bag received another overhaul, mainly a change in materials made them look more fluffy and modern. The flap colors were adjusted to reflect the newer frame colors of the bike, a couple of flaps for the S-Bag came and went. I own an S-Bag from 2010 or so and would not know a relevant thing to change. The O-bag was introduced in something like 2010 and never changed, apart from colors. The bag frame changed it's design from the original, full metal design to today's aluminium/plastic combo in about 2003. Long time ago. I do not miss a thing. Honestly, this is not in defense of Brompton. I just don't see value in changes to change something. I something is pretty well thought trough in the beginning there is not that much to be changed later. With the S-bag they obviously had some learnings in the beginning, those were reflected into production and that was that. The T-Bag had three different versions until today (including the original version in about 2000 or maybe even earlier). First revision in about 2009 (bags on the rear side changed as well as the logo, yeallow inner liner added), latest one in about 2016 (more modern optics, very slight design changes). So it is really not a fast pace Brompton changes something on existing bags but on the other hand the need seems not to be there. Why fix something that isn't broken? Maybe an attitude that does not fit our modern world full of yearly releases of expensive cell phones, but generally something I more like than dislike.
I'd asume the dealers are thankful for this attitude, too - they can stock bags and are less in danger needing to sell them off cheaper because of a yearly design change. with a dealer network as huge as Brompton's and the number of variants in bags alone this alone is an asset.