Right now, no matter how many mine, the block reward is the same.
Your formula would be difficult to implement and it's not future proof. After the last halving the miners will earn only from tx fees. How will you formula apply then? It won't. Sorry, IMHO it's not a good idea.
No more halving. The reward of bitcoins would be dynamic and thus the cap of 21 million would be lifted.
Many people argue that what gives Bitcoin its value is scarcity. If you remove bitcoins cap it would lose that benefit. You're system would also need a cap at some point to eliminate the infinite supply concept. Even governments use manipulation of the supply of money to control inflation and deflation. Your system would be continuously inflationary.