Does there have to be something else? What are the implications of a company growing forever? In 500 years are there going to be only 3 companies because they acquired everything and nothing else makes products?
So many people's first instinct to respond by buying more shares of that company, thereby driving its price and market cap higher. That's a reaction that implies that we think that the general trend is toward Buy-n-Large. If we expected regression to the mean to be the driving phenomenon, then we'd be more likely to respond by selling.
(Disclaimer: I'm not suggesting that's actually how things work, just that a lot of us behave as if we think that's how it works, or should work.)