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

Saifedean Ammous said:

If Saylor were to do something detrimental (for example, propose printing more BTC), the value of the BTC he holds would decrease, so he has no reason to do so.

The Bitcoin in the vault 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 self-protects: no individual or organization can easily manipulate or break the Bitcoin protocol.