Every buy of Bitcoin makes the Bitcoin in your possession more valuable. So the more they continue to buy, they more Bitcoin strengthens and by extension becomes better for those that already have some. You may be speaking from the angle of them creating monopoly which can be used to manipulate Bitcoin price. While this is possible in the future, it will not be easy for them to achieve because there is also associated risk to them and they are aware of it. Do not worry about their buys, just protect what you have and allow them continue to buy because their action will surely turn to your advantage.