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 make bitcoins cap we're removed 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.
If the adoption of bitcoin is not rapid enough, there won't be enough transaction fees to pay for the miner's electricity. But if it's too rapid, there won't be enough bitcoins for everyone (with the current system).
With my system, miners continue to earn bitcoins in perpetuity and more mining is encouraged and a growth that is too rapid is not a problem since the introduction of new bitcoins will be proportional to the growth.
Inflation is bad, but generally if it's less than 2% it's not a problem. The formula could be tweaked to achieve less than 2% inflation. Which even in an infinite time frame would be OK.
Bitcoin at it's inception had a 100% inflation ratio! The current inflation rate of bitcoin is about 4%/year, which is a bit high.