That very vague.... based on what?   The hard rules of the protocol are what protect the users and owners of Bitcoins from miners whos interests are only partially aligned.  Sadly, miners have substantial censoring power for data that goes into the blockchain.  I suppose it's useful to have an in-protocol way of coordinating rather than depending on potentially non-transparent back room dealing; but almost anything in the network would be easily gamable by miners. There are some things that I think are preferable to just having no effective limit (e.g. having a rolling median, and requiring mining at higher diff to move the needle for your own blocks, and requiring difficulty to not be falling over-all for the size to go up) but these don't address half the concerns and potentially add a fair bit of complexity (which has its own risks.).