Another concern of quantum computers is the effect it would have on Bitcoin difficulty and rising it so much that it would price out "normal" every day users out of the market causing a price crash in Bitcoin.
My proposal to the solution of difficulty increase is that we impose a hard restriction of the maximum amount of hashing rate per device.
what does price crash have to do difficulty rising? you do understand the most investors never bothered understand the meaning of mining in general and price is only driven by supply and demand? they could careless who mines bitcoin or how btc is being mined in the first place, so this statement is pretty wrong.
and how does a quantum based miner differ from a huge mining farm in China that cause spike in difficulty? there has never been a rule that says mining has to profitable or doable by everyone, it has always been a game where people with less power cost and more hash power dominate the mining industry, and this will always be case.
technically there is no way for us to have anything on the blokchain that will limit the hashrate for a certain device, the blockchain does not know where the hash comes from, this is how mining pools operate, everyone mining to that pool basically appears as single large miner on the blockchain. so such rule has no way to implemented neither is important.
this quantum computer fud is highly exaggerated when to comes to bitcoin, the media is fooling the people by presenting quantum as a deadly weapon to bitcoin when it really is not, i suggest you spend more time reading about what quantum is actually capable of doing.