There is no doubt in my mind that something like that is coming, it seems that people forget that during the Great Depression the US government confiscated the gold that was in private hands, there were many people that disobeyed the order and were able to keep their gold and get great profits thanks to it and while there is no doubt that gold is easier to confiscate than bitcoin, how many people are actually holding their bitcoin in wallets only they control?
It seems to me that a great deal of people are storing their coins in exchanges and if at some point a law like that is passed then they're going to lose all their bitcoin, they will be incredibly mad about it but at that point there is nothing to be done, this is why if you're going to use exchanges you need to use them for what they are for, trade there and then immediately get your coins out of there.
I also think it depends on the number or quantity of bitcoin that an investor have that might make them to hodl them in an exchange, to me that is. If I have a lot or some reasonable amount of Bitcoin, I wouldn't hodl those in an exchange but investors should only hodl some tradable amount of such assets in exchange just for trading in order to avoid such kind of losses.
I would not have any problem with someone that keeps a small amount of money in an exchange because he wants to trade or he is waiting for his orders to be filled, that is completely normal, the problem comes when people use the exchange wallet as their main wallet because they think it is more practical.
But then the exchange gets hacked or they close their account and they finally realize they never had coins to begin with, and I think there is the risk that governments will begin to confiscate the wealth of their citizens to pay their debts, and bitcoin is definitely an asset they will like to have at any cost and exchanges are an easy target for them to get all of those coins.