They will sell but won't sell all bitcoins they have in one day or in a few days. They will only do it if their company has big problems and accident. Else they will do it gradually like whales do distribution phase in the market.
The Bitcoin market has been becoming bigger and its trading volume growing with time so there will be better ability to absorb selling pressure from MicroStrategy and other institutional investors. Sells, news about it will cause fear on the market and price will be affected but it will give nightmare to gamblers with leverages and risk of forced liquidations. Oppositely, it will give great opportunities with big discount to people who are intelligent investors and wait for dips to enter.
Well there's a possibility of Saylor selling some of his Bitcoin but i don't think it would be anytime soon but in the future. He could decide to sell overtime, maybe if BTC reaches an all time high of $100k or above, over the years we've known him to be one of the major buyers of the asset and there's no point of him selling when the halving is by the corner. Instead MicroStrategy selling it's BTC they'll rather acquire more.
My opinion is based on his statement in a video I came across on the media, he was asked "at any point would you sell, when would it make sense to take profits there"? And he replied saying "well I've famously said I'm going to be buying the top forever. Bitcoin is the exit strategy, it is the strongest asset, so what we've seen right now, is the Bitcoin is just emerged as a trillion dollar ($1,000,000,000,000) asset class and it's along side names like Apple, Google, and Microsoft but the difference between Bitcoin and the magnificent seven is that, Bitcoin is an asset class it's not a company, there's not a lot enough room in the capital structure of those company to hold 10 trillion or a 100 trillion dollars worth of capital. So Bitcoin is competing against Gold which is 10x what it is right now, it is competing against S&P index, it competing against Real Estate a 100 trillion dollar plus asset class as a store value. So we believe capitals can keep flowing from those asset classes into Bitcoin cause Bitcoin is technically superior to those asset classes. And that being the case, there's just no reason to sell the winner to buy the loser".
With that being said, i don't think Michael Saylor would sell his Bitcoin anytime soon. I think he's noticed that Bitcoin is a better investment that would yield him more profits and that's why he's staking more instead of selling it. Well Michael has acquired wealth for him self and whether he sells or not is none of my business and I'll rather focus on working hard to acquire my own Bitcoin and prepare for the halving, the Bullrun is already started but it's not too late to acquire more Bitcoin before the halving.