There are going to be coins robbed, no doubt. However, I wouldn't take it for granted there will be millions. Sure, there are millions in P2PK, but perhaps they get spent until then; especially after the cryptographic community accepts some quantum safe alternative.
Most of which are lost, because people couldn't be bothered to have a backup for them. Satoshi is known to have a million Bitcoins at least, and there is probably more than that in terms of non-Satoshi but lost coins. In addition, there are also coins in exposed P2PKH addresses. These could add up to a few millions when the time comes. Of course, these are just vague estimations but that is more than likely to be the case.