"It's very rare for cartridges to just die. I still have folks asking for .38 S&W and .32 S&W Long enough to keep those loads in stock. As long as there are guns, there will be demand for the ammunition."
I will throw a big "AMEN!" to that one. Just browse the available brass at Midwayusa or Starline. I venture to guess that if it was a commercially produced cartridge in the 20th century, it's probably available.
A cartridge becomes obsolete when the gun manufacturers don't chamber it anymore and eventually the ammo manufacturers discontinue — pretty much in that order. Just because you can buy 43 Egyptian, 40-70 Sharps, 348 Win or 225 Win brass for instance does not mean the cartridge is not obsolete. There are a lot of companies specializing in obsolete brass that once only Bertram used to make and now you can find it from four or five suppliers. As a rule of thumb there are more rifle cartridges that fall away than handgun cartridges however there have been a lot of propriety and new designs in the past twenty years of so and I would think you will find a number of them going away sooner than later