That seems like a rather broad brush? Are you saying you can that you can point to a specific time in history where afterwords no new cartridges need to be made cause all possible cartridges for all possible uses exist?
IMHO I would say there are a fair number of new cartridges that bring new functional aspect to the table. Many are very niche cartridges but they are still useful to specific users.
I’d say.... probably after 2008. By that time most new cartridges were either reinventing the wheel or were marketed as “solutions” to “problems” that didn’t actually exist for most people.
To me that’s why so many new era invention cartridges/calibers die out in a relatively short time, because they aren’t all that different from what already exists.
There are some, a rare few that look redundant but actually fit a need. The 6.5cm being one. It has advantages over both the .260 rem and the 6.5x55
It’s no secret that manufactures make more on guns than they do on ammo. In order to stay in business they NEED to sell guns. While the tried and true will continue to sell, creating a new cartridge and marketing it well, creates new sales and THAT is where the money comes from. It doesn’t really matter if the cartridge doesn’t live on onto the next half century because they made their sales and have moved on to the next new thing.
If you can get a generation of shooters to believe that what they’re using is inferior many will ditch it for the new thing. That’s how cars, phones, and many other products, including guns, sell. Is what the buyer currently using inferior? Probably not. Can the user use it well enough to take full advantage of the new thing? Also probably not. But if you can CONVINCE them otherwise, neither of those points matter.
The most successful producers in a capitalist society can CREATE a demand where there wasn’t much of one already and then dominate the supply. It’s a brilliant strategy and works time and again.
For the vast majority of shooters, anything that needs to exist already does.
I haven’t a scrap of research other than my own limited experience to back that up.