Yeah it reduced as much people joined the network, I clearly stated it.
No, you didn't.
And the reward has nothing to do with more people joining the network, it's a protocol in the system; every 210,000 blocks, the coinbase reward is halved and it is currently at 6.25
BTC. You might want to correct the OP.
First, the increased use of bitcoin could reduce the cost of living for some people around the world. This would lead to an increase in standards of living for those who have access to this new form of currency.
Use of Bitcoin does not reduce cost of living. Bitcoin barely impacts the economy of a Nation, and cannot directly impact the life of holders, simply cause they hodl.
Second, the popularity of bitcoin could lead to an increase in wealth among those who own it. If you can invest your money in bitcoins and make a profit, then you have more money than ever before!
Third, there is also a possibility that new jobs will be created as companies begin accepting bitcoin payments.
The number 2 point is the most practical. But Bitcoin is already very popular, and I would not be expecting a demand shock due to increased population, which happens gradually overtime.
Bitcoin does not create new jobs either.