Am also afraid as you are because so many bitcoins will now be left in the hands of these institutional investors thereby leaving individuals with few or low bitcoins in their custody which means that we can just wake up to an unfavorable news especially when they start to consistently manipulate the market.
Regardless of the fact that you're speculating, I see some point in what you're saying, yes they'll definitely sale cause they can't hold forever but it's going to be a very long ride before they'll think of selling, I see Micro strategy as a company that's pictured what the future would look like with bitcoin and decided to pump lots of cash flow into it against the future, so i think they'll definitely sale but not anytime soon.
This year will be marking it's 4th year ever since Microstrategy bought their first Bitcoin of over 21,454 BTC in August 2020 at approximately $250 million, and as such, I don't think Microstrategy or it's CEO Michael J Saylor intend selling their portion of Bitcoin anytime soon, since we all know he is one dedicated Bitcoin maximalist who strongly believes in only Bitcoin and no other cryptocurrencies and likewise understands what the future holds for Bitcoin as a currency with the potential to dominate the decentralized digital currency space in the near future. So I don't think there is any need for anybody to start panicking about what could happen if these individuals someday decide to sell their portion of Bitcoin. Because for the fact that Bitcoin existed before Michael J Saylor bought his Bitcoin in August 2020, it will still be existing even after he sells his portion.