I like this a lot! It reminds me of some of the browser games I'd stay up late playing for hours as a kid. Contrary to what some other people here have said, I like that it starts out pretty bare bones. With simple game mechanics like this, it's really rewarding seeing the benefits of the upgrades you're able to make. It also feels very balanced with how the difficulty can still catch up with you despite the difficulty.
One of the first things I thought of was that the gameplay could be really taken up a notch (in fun and uniqueness) if there was sort of a path you had to follow. If there were sort of dense trees or other obstacles and a path that could go left to right, up or down, turns etc, it would add an extra layer that would add intensity. Just an idea!