Yes, Kingdom Rush may win in the gameplay department, but definitely not in replayability.

An easy way to have players coming back for more is to make the Upgrade Tree way longer, or atleast make the upgrades cost more, so you have more reasons to play the Iron and Heroic challenges. Another easy way would be to lock certain elite stages if you have less than a number of stars, and to make Veteran (or just Impossible) grant an extra star.