Most of Satoshi's block rewards are vulnerable; Bitcoin Core was configured for P2PK in the Coinbase transaction and that makes up for approximately a million (IIRC) of his coin or at least those that weren't moved or presumed lost/burned. Considering that each address has 50 coins, and at current prices, that would be north of 3 million per address. That is assuming Bitcoin doesn't grow further than 60K, which is hard to tell. The economic impact of losing that many coins won't bode well.
From a economic point of view, yeah that would likely have disastrous economic short term effects, but I'm not entirely sure that long term effects would be negative. If they were to be stolen, and we don't consider the morality in that right now, it would mean that there's more coins in circulation, which depending on what point of view your looking at might devalue the amount of coins, because as you know some people treat lost coins as "donations" to the network.
I'm still convinced, it will be a very long time until anyone with a motive to do it i.e common criminals, will be able to access quantum computers. I'm not convinced that the government, whatever that one might be would have any real motive. I think we like to think they're threatened by it, but I don't think in its current state it is.
I don't know the ideal solution, ideally we should never really assume coins that haven't been moved for a very long time, are "lost" so "burning" them without permission from its owner doesn't quite sit well with me. So, now to move onto the ethics of dealing with this scenario; is it better to burn the coins without permission in an attempt to better everyone else using the currency or is it better to allow those coins to be attacked, despite their owner having lost them, whether that's through choice, mistakes or not being around anymore. Its something which will need a decision, but quite frankly I'm not quite sure what's the correct way of going about it, obviously this has been brought up a lot over the years, and I think most would lean to the greater good, but I'm not a big fan of that viewpoint.
This is why I'm hoping people far brighter than me are in charge of this decision. I don't know the true economic damages that something like this would cause. If its just short term, which I believe short term would be the worse of the two, then that isn't so bad for a currency. If it has longer term effects, that could be detrimental to the survival of the currency (which I find implausible) then that's something that I imagine that needs to be taken into consideration. Even then though, burning coins that don't belong to you doesn't bode well with me. My question, and this might be more of a rhetorical question is; does anyone really know the true implications that it would have on Bitcoin?