Re: A different approach to Bitcoin's scalability issues?

Why not make the block size simply depend on some fraction of mempool size...

Simply stated, there is no "the mempool". Every node has a mempool and every mempool is assumed to be different.

Of course every miner will set mempool size to a different value before mining the next block.

A node cannot validate the size of the miner's mempool, and if the miner can choose any size, then there is no point.

There is no need to validate any precise value that will change anyway. When averaging over 2016 blocks the result will get close to average mempool size that other nodes see.

There is no point in having a requirement that does not have to be followed.