You've posed a very good question. We've seen some shells discontinued recently and I suspect that we might just see more in the future. With the rising price of components, especially tungsten, which is a part of the shot formula for many manufacturers, we may see some makers quit making shells with that shot. We may also see a reduction in the amount of tungsten in the shot of some of those shells.
Recently, at least one company, has "re-branded" some of their very inexpensive lead shot shells in an effort to move some shooters/hunters to their brand. We could and probably will see more of that activity, if the overall results were favorable.
It's really kind of a tough decision to make but maybe looking back a few years will be of some help.
In 1999, I was shooting 3.5" lead loads for turkeys and 3.5" steel loads for waterfowl. Everyone I knew was doing the same. When that radicaly product Hevi-Shot was introduced in 2000, I really wanted nothing to do with it. Experts at that time said it might/would damage your barrel because it was so hard and the shot just looked funny, it wan't round and shiny, so it couldn't possibly fly straight. Some of you older shooters/hunters may remember all of that conversation.
After 2000 and for the past decade+ I've been shooting Hevi-Shot products and other tungsten-based shot for turkeys in mostly 3" shells. Even steel shot has improved so much over the years that I would not even consider shooting a 3.5" shell for waterfowl.
Now we're in another era which included price increases, again, and some manufacturers have decided that their products would be too expensive for us as turkey hunters to afford. That may be the case and if you have a gun that just loves one of those products, it may serve you well to stock up on those shells, if you can still find them.
It may also be the case that something new is coming down the road that we've never anticipated. I recently shot some of the Hevi-Metal turkey loads and was very surprised by what I saw from them. We are an innovative people and I look forward to seeing what's next.
"If he's out of range, it just means that he has another day and so do you."