In 5 years I am sure it'd be in the high trillions, maybe even quadrillions.
Have you taken into account lessening block rewards (I.E. block halving)? Miners will be persuaded to stop mining or to leave to an altcoin.
mining is always going to be a cat chasing its own tail game, the higher the difficulty, the more miners, the more miners the higher the difficulty.. (another analogy is that miners are always shooting themselves in their foot out of greed)
block halving makes weak miners who cant afford to continually upgrade, give up for a few months. and those big players continue on. what also happens is that the price of bitcoin rises (miners don't sell at a loss, causing a low supply, high demand price rise) to a level that miners are happy to sell at. again some miners will give up as they cant keep using their FIAT reserves to pay the electric.
but all in all the miners vs difficulty will keep rising.
this cat and mouse game has been going on for 5 years,
with the movement from CPU to GPU
with the movement from GPU to FPGA
with the movement from FPGA to ASIC 110nm
with the movement from ASIC 110nm to ASIC 55mn
with the movement from ASIC 55mn to smaller nm (faster asics) to smaller nm (faster asics) to smaller nm (faster asics)
not even the last halving stopped any of this.
the greed of miners will ensure that no matter what, there will never be a time that people can start up their old rigs and be able to mine like they did 'in the goold old days' of first having that rig.
