I merited your post because I think it is good to discuss these types of ideas, even if they are bad. These discussions could potentially lead to good ideas.
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Ok let's do this
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 so they have to accept other people's transactions to maximize their income. Also, I don't see why a miner would purposely raise his own storage cost...