In the day, cars had maybe on upgrade in trim. You got Standard, the base model, or Deluxe, cause you could afford to put on the dog and the extra chrome or AM radio and heater was nice.
By the 80's, options on cars were becoming so complex it actually intruded on emissions controls and the factories were seeing cost rise too high. You could get 5 or 6 trim packages. They continue to respond to every custom car fad and trim option that becomes a show car necessity by adopting it and making it part of the standard. It's actually repressed the aftermarket a bit. Last years model was tricked up with ebay taillights with LEDs, this year's, they are factory standard and actually look nice.
Same with guns. Ok, so SIG is doing that. The P938 is a good example.
http://www.sigsauer.com/CatalogProductList/pistols-p938.aspx Ten different trim options on the same gun ( I wouldn't call it ten models, tho.) And even that has changed up on some over the last two years. In the old days, there would have been just the one model in whatever finish and grips, take it or leave it. After that, the customizing would start up, different sights, grips, refinish, melt job, etc. What SIG did was give the consumer all that up front and get their money - up front - at the retail level. While a few are going to retro fitting triggers and the vendors are certainly out there with over the top "trucker girl" grips, SIG has at least offered a custom touch to the guns available so that the consumer won't have to do much more after the purchase. That draws more customers who see what they like and prompts more sales.
We are also talking a much bigger market than the '60s or '70s when we hadn't reached 200 million population. Now it's past 310 million. That is a lot of new customers out there in just 40 years, about 25 million more. Those kinds of figures mean enough more market, and with computerization and improved logistics and production programming, you can control more options with less costs. Just in Time shipping for production allows smaller batches of parts on hand so their is less money tied up in unsold product waiting to be assembled - or a trim option dogging in sales and sucking up capital.
It's going to keep expanding as new items and features come into the public's eye. Take a look at black palm grips - I see an opportunity for someone to come up with a synthetic substitute for the natural weave pattern in the grain, and that there are plenty of other things being churned by gunsmiths that could be copied to make new models.
American business does that - we might complain about cheap offshore copies, but the real business of America is making a variation of what the other guy is selling so we can get our share of the market. Jeans, tactical knives, cell phones - it's right out there in our face every day.
If you want to sell athletic shoes right now, just put springs in the heels, use a woven web fabric in a neon color, add numerous sewn on reinforcements. What Brand did I just describe? No, not just One Model Only, you can find about a dozen.
Same with guns. WE are making that happen because WE are buying them. The gun makers are just responding to how we are throwing our cash around. Did Remington come out with a retro auto? Yep. Demand. I suspect a new version of the P7 could do well.
More customers = more money = more trims and models.
Not to discount another trend, the Gun of the Month club, which is about fashion and having something new you don't. Even more sales.