Your suggestion effectively allows the miners to decide the blocksize. It would be trivial and cost-free for the miners to either send many transactions to themselves with an arbitrary transaction fee, or to include fewer transactions than is economically logical.
Actually, there is the cost of opportunity because miners don't get paid mining their own transactions. Also, I don't see why a miner would purposely raise his own storage cost...
A miner does not get paid to confirm his own transactions, but he also does not pay anything to confirm his own transaction. If the maximum block size is based on the last x number of blocks, including additional transactions will increase the maximum block size in the future. There is some opportunity cost, in the form of the greater chance that a block will be orphaned when it includes an additional transaction.
Let's stop playing I think you know exactly what I meant. Let's just say it explicitly for the crowd, if the demand for block space exceeds the supply then you'd rather mine high paying
real transactions than your own
no paying fake transactions.
The additional storage cost of including a transaction is close to zero.
Does that mean there is no miracle solution to spams?