Is there any way to know the amount of coin in the largest bitcoin wallet ?
If a big player manage to accumulate more than half of the coin supply, it would mean that he will have a lot of power to change the value of the coin. It would mean Bitcoin would still have a damocles sword above it.
Am I delusional or is it a valid concern (even if it's unlikely) ?
I'd say that you shouldn't worry about it. Assuming that someone has the half of the coin supply, why would he even dump it all? Dumping it all at once will destroy the market, plus, another whale can just save the whole market and end up buying at a really really low price. Think of yourself as the guy that has half of the coin supply, you would rather to sell every now and then when you to spend some money. You'd be holding those coins forever, I promise. Since you'd want to hold them as long as possible to get the most profit out of it.