As it becomes more difficult to mine a bitcoin older technology becomes obsolete due to difficulty as does falling BTC to fiat ratio. What is going to stop this next downtrend if there is nowhere really to go for the miners.
Bitcoin farms will be unplugged. No more bitcoins but users still trading. So more users want bitcoin but less are being produced.
From CPU to GPU to ASIC, the mining efficiency has been going up incredibly, but the improvement rate can't be sustained like that forever as the new generation of ASIC are already in 20nm process.
Also, as torusJKL explained, if some miners quit, the difficulty will adjust itself within 2016 blocks.