I like your proposal. I've never been against the increase in the block size but always believed, and still do, that it should be more dynamic in nature and follow on actual bitcoin usage instead of it being based on how we think/perceive bitcoin to be used in future.

If it's not dynamic in nature we are bound to run into problems again in future if we say for example double up on the block size only to find out that mass adoption is not happening at the rate we expected. With your proposed solution both sides are covered as the block size will dynamically increase and or decrease based on actual usage. If properly implemented it could mean that we can lay the arguments around block sizes to rest and never need to worry about it becoming an issue again in a couple of years time.