I think that a really conservative automatic increase would be OK, but 50% yearly sounds too high to me. If this happens to exceed some residential ISP's actual bandwidth growth, then eventually that ISP's customers will be unable to be full nodes unless they pay for a much more expensive Internet connection. The idea of this sort of situation really concerns me, especially since the loss of full nodes would likely be gradual and easy to ignore until after it becomes very difficult to correct.
As I mentioned on Reddit, I'm also not 100% sure that I agree with your proposed starting point of 50% of a hobbyist-level Internet connection. This seems somewhat burdensome for individuals. It's entirely possible that Bitcoin can be secure without a lot of individuals running full nodes, but I'm not sure about this.
Would 40% initial size and growth make you support the proposal?
40%/year = 96%/(2 years). I hope that gets rounded to "double once every 2 years (105 000 blocks)".
Also, do you propose this growth be open-ended or terminate at some block-size commensurate with the transaction volume of an existing, mature payment system such as Visa? Given an open-ended proposal I'd echo Theymos' concern.