Will lack of content and slow development kill this game?

The short answer is yes. And a cap increase and additional levels (8) which essentially just become more places to farm, do not equal new content. New content would be anything the OP referenced. I know this game is "early" in terms of its overall development. But as the famous Yogi Berra would tell you, "it's getting late early."

I want new ways to use my roster of toons, not just grinding them to higher levels. I like my characters, let me play with my toys!!!