The biggest crypto fund Grayscale Bitcoin Trust bought another 15 114 BTC last week and fund's crypto AUM is almost $10B with 506 000 BTC. In my opinion such BTC stockpiling can be dangerous like a 51% attack but I didn't find any information is Satoshi Nakomoto ever mentioned that.
There's two things I would like to mention
1. Holding a big amount of bitcoins doesn't mean the owner of those coins is able to do a 51% attack
But yeah, holding a big amount of bitcoins does give it's owner the ability to manipulate the price of bitcoin by doing a pump/dump.
2. In order to perform a 51% attack the individual/company must own 51% of hashing power used to mine bitcoins
In other words it means that the individual/company must have loads of ASICS so that their hashrate increases and crosses 51% of the total hashrate.
Also, even if anybody has the ability to perform a 51% attack, doing do won't do him any good because the cost of doing such an attack is a lot.
Basically, Satoshi had designed the bitcoin network in such a way that it will become very hard for anybody to compromise it in any way.