It's also pretty much impossible to CPU mine anymore, and with the addition of ASICs to the mining pool, even running a GPU miner is no longer profitable after calculating electricity costs.
I understand that you don't have to be mining to assist in verifying transactions, as simply running the bitcoin-qt client aids processing of transaction, but there no longer is a financial motivation to do so.
With an increase in the rate of transactions as the Bitcoin market continues to grow, it seems that the transaction speed will certainly slow down, and the security of the network will decrease.
If there is anyone here that can explain to me why I shouldn't be worried about this situation, I'd absolutely love to hear it. I'm bullish on Bitcoin, and I think there is still a good future for the currency, but I'm worried...
If there were to be fewer miners to the extent that transaction time becomes a problem, people will attach higher fees to transactions to get them to complete quicker, which increases the profit from mining, which makes more people want to mine, etc. etc.
Indeed, what will happen if most of the people end up firing up their bitcoin-qt client only to do a transaction, and shut it down immediately after?
There won't be an "exponential drop" in mining activity, because mining rewards exponentially decay anyway, so the system involves no sudden changes at all.