The fixed 20mb cap isn't actually my proposal at all, it is from Gavin. I am supporting it because anything is better than nothing. Gavin originally proposed the block size be a function of time. That got dropped, I suppose to make the process of getting consensus easier. It is "the simplest thing that can possibly work".

I would like to see the process of chain forking becoming less traumatic. I remember Gavin, Jeff and I once considered (on stage at a conference??) that maybe there should be a scheduled fork every year, so people know when to expect them.

If everything goes well, I see no reason why 20mb would be the limit forever.