It doesn't work that way.
Difficulty is a lagging indicator of global hashrate. If hashrate goes up, difficulty proportionately increases to compensate so as to keep production of coins at a constant rate. Who controls the difficulty resets? The program itself controls it--it's built into the software and nobody can change it short of getting a majority of the entire network to agree to a software change. As it stands, the software simply adapts to hashrate and artificially increases/decreases difficulty to maintain a constant rate of production.
The 50 to 25 thing is different; it's built into the algorithm that every 4 years, the reward per block halves. So for ~4 years, bitcoin was produced at a rate of 50 coins per block, one block per 10 minutes on average. Right now the block reward is 25 coins per block. 3 years from now it'll be 12.5 coins per block, and so on and so forth. This restriction on coin production is ALSO built into the software as a means of artificially tapering down supply so that by 2100 or whatever, we will basically have a fixed supply of bitcoins since block rewards will be tiny.
Also, profit drops faster than revenue if you pay more than zero for electricity. A thought experiment for illustrative purposes (these are not actual numbers): say it costs you $1 in power to generate one coin worth $3. If the difficulty doubles, it means you generate only half a coin, which is worth $1.50. So before your profit margin was $2, and now it's $0.50. Difficulty doubled and thus your revenue fell 50%, but your profit fell 75%. This is something people seem to miss--a doubling of revenue results in the loss of more than half of profit, unless you have free electricity.
Regardless of electricity costs, there are opportunity costs (could invest the money elsewhere, such as by directly buying coins instead) and depreciation costs--every day, computer equipment loses some of its value. Just like how a $600 cell phone today will be worth almost nothing in 3 years. And of course the risk of hardware malfunction, a PSU blowing out and starting a fire, etc. If you're going to mine, do it with premium equipment that won't burn down your home, that is for sure.