The thing with limited art prints, like limited anything, is that if 30 or 50 or 100 or whatever are printed now, 50 years from now the same number will still be around. Theatrical release posters, however, will dwindle over the years as they are destroyed in the regular course of business. Some get caught up in collectors' hands, true, but even those dwindle as they are sold, lost, destroyed, or whatever. The point is, a collectible print will always be protected from destruction and hence the numbers will not winnow over time as they will with regular posters.