[simias]

But the more popular the coin becomes the harder it becomes to mine it. If the increase in mining difficulty (driven by total network hashrate) increases faster than the price of bitcoin then your mining apparatus becomes less and less profitable.

As for repurposing hardware, that might work with GPUs (although their value depreciates pretty quickly) but the Antminer is a custom ASIC, so it can't do a lot more than bruteforce SHA-256.