When quantum computers reach a point to be of use in mining, then the security algorithm will be upgraded too. If that proves impossible, then Bitcoin will be dumped for a new more secure coin. The market will adapt to the technology, but only when necessary.
ColdMonkey mines Gridcoins through BOINC computations for science…
Unfortunately, that is how things typically work. This is a subject being discussed in many crypto-communities, but I don't know of any with actual plans to upgrade their security to prepare. It may be too complex to upgrade, and require a rewrite to incorporate. I think it is going to be a consideration more people take into account when looking into their altcoin investments.
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