Is Gavin saying this should grow at 50% per year because bandwidth has been increasing at this rate in the past? Might it not be safer to choose a rate lower than historic bandwidth growth? Also how do we know this high growth in bandwidth will continue?
Yes, that is what I am saying.
"Safer" : there are two competing threats here: raise the block size too slowly and you discourage transactions and increase their price. The danger is Bitcoin becomes irrelevant for anything besides huge transactions, and is used only by big corporations and is too expensive for individuals. Hurray, we just reinvented the SWIFT or ACH systems.
Raise it too quickly and it gets too expensive for ordinary people to run full nodes.
So I'm saying: the future is uncertain, but there is a clear trend. Lets follow that trend, because it is the best predictor of what will happen that we have.
If the experts are wrong, and bandwidth growth (or CPU growth or memory growth or whatever) slows or stops in ten years, then fine: change the largest-block-I'll-accept formula. Lowering the maximum is easier than raising it (lowering is a soft-forking change that would only affect stubborn miners who insisted on creating larger-than-what-the-majority-wants blocks).
RE: a quick fix like doubling the size:
Why doubling? Please don't be lazy, at least do some back-of-the-envelope calculations to justify your numbers (to save you some work: the average Bitcoin transaction is about 250 bytes big). The typical broadband home internet connection can support much larger blocks today.