Anyone who thinks the hellcat cars have anything to do with Viper slow sales are, well, to put it bluntly, exhibiting a laughable lack of understanding.
Firstly, other than being 600hp plus cars, they aren't the same demographic at all. No one looking at a Viper is going to say "hey, that hellcat has more hp, it must be a better sports car". That's just laughable. Second, Viper sales have always been crap. They've never been good. They release a new model, sell a couple thousand over a few years, and demand dies out. It's like the Corvette sales pattern, but over 50 times smaller. New car is released. Sales peak. Sales then taper off to about a quarter of the first year model sales after 5 or 6 years. This works for the Corvette because they sell enough to justify the R&D for the next model. For the Viper, the absolute best they can realistically hope for is pushing 5,000 cars throughout the whole run of a given design. It's always been that way. Actually, we might say 3,000 cars is the best they can hope for with a design. This makes absolutely no sense for a company like Dodge to do, especially now that the car is becoming an also ran in the high hp sports car arena. It sold marginally better when it seemed bonkers in comparison to all the competition. It really was a class of its own. Now though, in a sea of "similar" options (AMG GT, F-type, Z06, etc), it gets lost.