I think the spring breakage issue on the 'demo' knives is just an issue of quality of the heat treat, and perhaps a poor
grade of stainless steel. I don't believe it's just a generic stainless vs. carbon steel thing. The overwhelming majority of current production knives are made with stainless springs these days (all Case Tru-Sharp knives, and Buck's entire line, among many others). Rarely do they break. Most of the production folders I've seen with broken springs are older ones with carbon springs anyway.
I think the whole notion of the 'demo knives' being built to 'Mil-Spec' is somewhat overblown, or perceived as meaning more than it really does, in terms of quality. Sometimes, 'Mil-Spec' is just a basic list of desired features on a tool, as opposed to a stringent checklist of QC requirements. I'd bet the overriding concern with the Demo Knife was a requirement that it won't rust, hence the all-stainless construction. These knives have always been produced at minimal cost, to do a relatively simple job. Same applies to at least a few versions of the TL-29 knife. A friend of mine, years ago, spent 10 years in the Navy during the Vietnam-war era. He spent most of that time on aircraft carriers. He had told me, in somewhat 'embellished' fashion (

), that his TL-29 would sharpen up quickly, and lose that edge by the time he'd walked across the flight deck. I'm sure it wasn't quite that bad, but I understood what he meant. It's an inexpensive, basic tool. Not too much is expected of it.
A lot of older knives show signs of stress near the anchor pin for the backspring. Most of the time, it shows up as cracked scales near the pin. Sometimes, when watching closely as the knife is opened & closed, you can see the brass-lined frame flex and/or twist. Lots of movement around that pin, which is usually why the scales crack, especially with very brittle bone. Take a similarly constructed, cheaply-made knife made with steel liners/scales & pins, which presumably wouldn't flex as much as thin brass, and a lot of the stress at that joint will be focused back on the anchor pin or the backspring itself. A cheaply-made spring, of any steel type, might not take the load too well. The stress is compounded on a spring that gets flexed from both ends, such as found on the 4-bladed demo knife (vs. the 'TL-29' style, which is only getting flexed from one end).