Ideally the best block difficulty is one that is extremely high so as to protect the block chain.
However, without adequate computing power an extremely high difficulty would make block generation take far too long.
So the difficulty is adjusted periodically to allow available computing power to generate a block in about 10 minutes.
The sole reason for changing block difficulty is to adjust for computing power available.
However computing power available does not readily equate to economic activity.
Initially it may to a great degree. Miners will come and go as the price rises and falls, but over time inefficient miners will be driven from the market by competition.
Left standing will be miners who have paid off hardware and low cost or free electricity who will stay and mine regardless of the block reward. Even if all they earn is transaction fees.
The block reward should be based on the value of altcoin.
If the value rises the block rewards continue to increase until the value returns to the set level.
If the value falls the block rewards continue to decrease (even to 0) until the altcoin value returns to the set level.

The issue is what to use to determine the value of altcoin.
It needs to be something that is constant.

For example, if the USD is used and  1 USD = 1 ATC
and the USD falls in value it will look as if ATC is increasing in value, but really it's just the USD losing value.
But block chain rewards would increase thereby decreasing the value of ATC to bring it in line with the devalued dollar.
Gold wouldn't work well either as the price fluctuates greatly due to varying demand as well as the values of the currencies used to purchase it.