The decline I've witnessed has been the decrease in opportunities to participate in even semi-organized get-togethers where like-minded folks had fun. Exclusion of modern designs will continue to shrink interest simply because the availability of traditionals we once enjoyed is gone.
The person who wishes to shoot with a group of muzzleloaders but only has a modern design not allowed to participate is one more we lose. Certainly not to suggest allowing "moderns" to take over contests, but having a side event allowing them makes more sense to me than watching a man turned away to never be seen again.
One of the interesting aspects of Friendship to me were the custom guns competitors showed up with that adapted flint/percussion to modern designs, fine machine work, beautiful wood, great optics, etc. Those guns weren't meant to represent any "historic" period, but the results impressed me to this day.
Fair to suggest anyone wishing to purchase a traditional M/L at the largest retail firearms sellers - Bass Pro, Cabellas, etc. is out of luck.