With regard to Quantum Computers..
http://www.techworm.net/2016/05/ibm-launches-free-quantum-computing-for-everyone-to-use.htmlSo we are inching closer..
For signatures, it requires a change of the signature algo to a hash bashed/ quantum secure scheme, and if I'm not mistaken, segwit makes this 'easier' to do ?
As for mining, Grover's algo only halves the difficulty of a pre-image attack, which is what you need for mining, so a switch to SHA512 would seem to fix that..
Getting the current miners to switch to a different 'ASIC-Chip'.. (can you use old sha256 chips to mine sha512 ?) .. would be interesting.. As it could reset the current balance of mining power.. And we may get a second shot at getting mining 'right'.
There's hope yet, gentlemen.