There are always people who claim that a round that isn't in the top couple spots on the new sales list is "dying/dead/obsolescent/moribund,"etc.
Looking only at sales is comparing apples to everything else in the garden, which is hardly a fair way to do it, in my opinion.
Sure, Ruger wanted his new design to sell, and chambering it in the most popular and common revolver round at the time was simply good business. Note that the next round put into the Blackhawk was the .44 Magnum, when it was brand new, and unknown. Ruger actually got their pistol on the market before S&W did.
I'll happily take your poll, but my answer isn't going to be .45acp.
I will freely admit that when I chose my first Blackhawk .45 my idea was to get something that would let me plink with my .45 auto ammo, and not have to chase the brass in the weeds to find it for reloading. I had a .45 auto pistol at the time, but little experience with SA revolvers and no experience with the .45 Colt round.
The year was 1983, and I bought a brand new Ruger Blackhawk .45 convertible with a 7.5" barrel. My intent was to use it to shoot .45acp ammo, but on the way home with it, I realized that I ought to get at least some .45 Colt ammo for it, so I stopped at another place and bought one box of Winchester 255gr ammo.
When I went to shoot the gun for the first time, I made one "small mistake" that changed my mind completely. The first ammo I shot was .45 Colt. WOW!!
!!
The gun roared, bucked in my hands and the muzzle pointed to the sky!!! It was GREAT!!! I was hooked, and one might even say in love! After that shooting .45acp in that gun was just "meh". Over the years since, I've only put a bit over 300rnds of ACP though the gun, but many thousands of .45 Colt rounds. I eventually got a second convertible blackhawk, which has the ACP cylinder in it, and rarely leaves the case, I just don't use it, and half a dozen Vaqueros in different finishes and barrel lengths, ALL of them in .45 Colt only.
And, of all of them, the one I shoot the most is my first one, with the .45 Colt cylinder in it.
For me, its like the .41 Mag is for its fans, it does what I want, I enjoy shooting it, and don't give a rodent's posterior about where it is on the sales list, as long as it IS for sale, (ammo /components) I'm very happy with it.