A quantum secure network / addresses are possible and not an issue. But what will happen to the 'shalecoins'?
Maybe some will say to burn them in the new quantum secure network, what would lead to a fork.
And some would still use the new quantum secure network with all the old coins.
Ad 1. If quantum computing managed to break private key cryptography in a way that it allowed somebody to control shalecoins, this would instantly induce a panic which would crash the price of bitcoin, so it wouldn't be $1,000,000,000,000 anymore, more like $1,000.
And the Bitcoin ecosystem would be broken. The Bitcoin community should not allow this and we have to solve that problem now.
The best solution would be, somebody could move the 'shalecoins' to P2PKH addresses.