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) ?
In fact, there is. It's by using google. Anyway, there you go: http://bitcoinrichlist.com/top100 I simply googled "Bitcoin wallet with most coins." and there it is.
If someone owns more than half of the coins supply, I doubt that he'll put it in a single wallet/address. Firstly, questions like yours will raise. Secondly, just so he can separate the coins and their wallet seed's stashes. If ever someone founds one of the treasures, there would be another one.
Honestly, no. You are not being delusional. This is totally possible but that doesn't mean that it's actually true. A lot of conspiracy theory can be made from anything, in this case bitcoin. Some people thinks that the government is secretly running this, others think that it's a huge ponzi scam, others think that a major portion of the coin already belongs to one person or group.