Being very deliberate about making sweeping changes to an application with an extremely large number of very satisfied users is not the same as allowing it to stagnate. Having spent 4 years of my life working at 37signals I know first hand the incredible amount of energy that is devoted to it by an extremely talented team.
That said, it's certainly not for everyone. And if you outgrow it, fantastic, feel free to move onto a new product that suits you better. We do this with many other aspects of our lives, why should software be any different?