However I don't see why not to start and think about how to implement potential quantum resistant algorithms to be used within the BTC blockchain. I know some altcoins already have implemented some sort of quantum resistant algorithms.
It is because there is no quantum computer and yet advanced one that will exist for now (and estimated to 2030 or more) that will be a threat to bitcoin. Although they will later be produced but when the right time will come, bitcoin developers will do something that will make it impossible for the quantum computers not to access people's private keys. They still know that it is not yet necessary for such upgrade yet.
And even if said computer would be created, its target wouldn't be BTC, first and foremost.
They've got much and much bigger fish to fry if it will become available to a broader audience, so to speak, said computers won't become a widespread commodity in a moment.