If you aren't going to run with the herd, you have to pay the price to break your own trail.
The last 100+ years have shown us a number of rounds that bloomed, flourished for a time, and "died", and yet are still with us. Some are more with us today than at any time in the past 50 years. Other are not, but everything is, and will be available, IF you are willing to pay the price.
And that's the rub. Most of us can't or won't. Niche, obsolete, failed, all these can still be had, if you can pay. and pay A LOT.
SO, the question is not so much what will "last", its what can I afford to feed in the worst case? This is where handloading comes to the top of the list. If you want something you feel is going to eventually be dropped by the factories, you BECOME the factory.
Pony up the price for the reloading tools and components now, and you will be set for the rest of your life (if you do it right, and we can talk about that, if you want).
If you are going to stick with what you can find on the local shelves, (or major internet suppliers), for whatever reason, then you are better off to run with the herd.
Niche round also includes rounds that are in current production, just not widely popular for general use. .22 Hornet is considered a niche round. Been alive and kicking since 1930, still is, but its never going to be cheap and abundant. specialty round, and specialty price.
The .357Sig is going to be with us a long time, even after every police dept stops using it. Might get scarce but its not likely to go away.
The .45GAP isn't likely to stay, once Glock decides its no longer cost effective to support it. It doesn't have even a fraction of the popularity of the .357 Sig. On the other hand, when they do drop it, any one with a stock of the ammo left after the "panic buy" rush when it does get dropped will likely have it for some time.
I might be a pessimist, but the rounds I would really avoid are the rimfire ones. The .17s are neat, but will they last? or will just one last? I remember the 5mm Remington very clearly. Last I heard a box of 5mm Rem was worth more than the rifle.
Can't reload the rimfires, can't make brass from other cases, when they get dropped, its hail and farewell as existing ammo dwindles and passes into the realm of too collectable to shoot.