Saifedean Ammous, the author of The Bitcoin Standard, argues that Bitcoin is not threatened even if Michael Saylor (Strategy) hypothetically accumulates up to 10 million BTC, nearly half of the supply:

Saifedean Ammous said:

If Saylor does something detrimental (for example, proposes to print more BTC), the value of the BTC he is holding will decrease, so he has no reason to do so.

Bitcoin in the custody of Strategy or BlackRock's ETF actually belongs to the shareholders and investors. If the two companies abuse their rights, the shareholders/ETF holders will sell out and move to another channel.

The market mechanism protects itself: no individual or organization can easily manipulate or break the Bitcoin protocol.