Is oversimplifying the situation and providing a false sense of security.
You can't just upgrade the $BTC software and consider the threat handled. It doesn't work like that. This is not a "flip the switch and it's fixed" kind of thing.
Even with a quantum software solution in place, EVERY USER OF A SELF-CUSTODIAL ADDRESS (WALLET) MUST MOVE THEIR COINS TO A NEW QUANTUM-SAFE ADDRESS, BY SIGNING A NEW TRANSACTION USING THEIR PRIVATE KEY.
Read that again.
That involves a direct user action to sign a transaction for every address they control.
Any address that doesn't do this would lose its coins to quantum computers and those coins would be likely dumped on the market.
The problem is: there are many millions of coins that it's impossible to move because the private keys have been lost. Think of the millions of lost and unmoved coins (such as Satoshi's wallets). These coins could, and likely would, be stolen by quantum computers and dumped onto the market.
One solution that works for KNOWN lost coins (Satoshi's, etc.) is for them to be locked via a hard fork, so they can't be dumped onto the market.
But that doesn't work for all lost coins because we don't know the addresses of all lost coins. How do you prove it's lost? What if it's just been in cold storage for 10-15 years? So, you can't just go locking addresses that appear dormant, because that's potentially tantamount to burning someone's savings.
Bottom line:
There are some big decisions for Bitcoin developers to make here. There are trade-offs with those decisions.
My best idea for a solution:
Developers set a cut-off date in the future (ideally far in the future). This would be a globally known date and communicated widely, where a hard fork would occur and, after that, any coins that have not yet been moved are FROZEN.
If you can pull that all off in time, then you can at least be on the defensive. We need this to happen before quantum computers can crack anything, because missing that date would leave the entire chain basically compromised. And this still REQUIRES ALL SELF-CUSTODIAL USERS TO MOVE THEIR COINS TO A NEW WALLET BY SIGNING A TRANSACTION. As such, with such a large-scale plan, there WILL be collateral damage. The aim is to minimize this damage. For example, there will be those who simply were unable to move their coins in the allotted time, for whatever reason-- perhaps a user's coins are in cold storage 3,000 miles away from them and they can't get to them in time, etc. Maybe someone is hospitalized and can't get to their private keys in time. Maybe someone almost remembers their seed phrase that they memorized (brain wallet) but only needs the last 3 words, and they can probably remember them if they just have a little more time, etc. The number of scenarios is unlimited. This is why MAXIMUM TRANSITION TIME is needed before coins are frozen.
A time-based hard fork solution must be put in place SOON, TO GIVE USERS ENOUGH TIME TO RESPONSIBLY MOVE THEIR COINS! Ideally, users have MANY YEARS to do this, so it's not some last-minute rush where many people miss out and havoc is wreaked due to the lack of time.
But it's starting to feel like it's going to be a last-minute rush if quantum is accelerating at the pace that Google's recent announcement makes it sound like it is. Regardless, it's always better to be ready sooner than later. There's no good reason we shouldn't already be migrating our coins, today. It's lack of planning and lack of foresight. The new wallet address format could already be implemented in many wallets and the migration could be beginning already. Right now it feels like Core devs are already behind on this because USERS GLOBALLY NEED TIME (IDEALLY, YEARS) TO MOVE THEIR COINS.
One cynical thought is: Core devs, compromised via Epstein or external funding sources, could intentionally wait too late (perhaps as they're already doing?), which would cause maximum damage to the chain. Speed of solution is everything.
Side thought regarding Altcoins:
You have to realize that there will be an enormous number of altcoins that WILL NOT BE prepared and do not have any quantum solution in place. Not every dev knows how to solve this problem, or will do so correctly. Those altcoins will all be drained to ZERO as soon as quantum computing is able to do so. So, even if BTC isn't cracked, the mass sell-off of altcoins will also put a lot of downward pressure on the overall crypto space as money flees from any quantum exploit. It could be a rough period, even if BTC gets it right.
Positive ending:
In the end, the dust will settle and crypto will still exist. Crypto isn't going anywhere. This is a major speed bump, but one that crypto will overcome. There could be some enormous volatility while the dust settles. That might even be an understatement. But, ultimately, crypto will live on and be even stronger in the future decades.
TL;DR: Devs need to get the solution implemented YESTERDAY because ALL USERS need to take action with their private keys.