Ironically if they start selling their shares would suffer more
Meaning their investors would suffer more than people holding Bitcoin.
They have more to loss than hodlers.
When the share rises an investor can choose to sell. That's different from strategy selling Bitcoin.
They won't keep holding forever, will they? At some point, MicroStrategy will eventually sell portions of their Bitcoin, maybe not all at once, but gradually over time. Of course, they won't just sell because they need to. They're more likely to wait for a new ATH to maximize returns. After all, their stock performance is now heavily tied to Bitcoin's price, so they will carefully time any liquidation to avoid negatively impacting their own shares.
Its hard to say what will happen when they start to sell. Impossible to predict until it happens. It also matters whether he starts selling in a bullish period or bearish.
If he starts to sell, panic will happen and others will sell also because they worry about the price dropping due to sell pressure, so their balance sheet will go down in value and so will their stock.
But when he sells, for fiat, that fiat wont lose value. So if he sold a large portion, bitcoin might of dropped but the value of his company can still remain high because he took profit basically.