Personally, I like that they're coming out slowly since they're so expensive.
There was a recent podcast featuring the guy in charge of figures at Mondo and he explained that they're a small company hence why it takes them so long to get figures out. He also explained that they only have plans for those characters shown right now (Batman, Freeze, Catwoman, Batgirl, Robin, Harley, Joker, Batman Beyond). He says there's always a possibility of more after that depending on sales and demand, but right now they only plan to make those figures previously mentioned.
As for wether the line will continue to be popular or not two or three years from now, then all I can say is yes, of course it will. It's Batman The Animated Series!