publicly traded companies are not "required" to make constant year over year gains for shareholders and investors, that is just what the owners usually decide to tell the company to do. The owners of a privately traded company could decide to, and the owners of a publicly traded company could decide not to. For example, zuckerberg controls 53% of the voting stock of facebook, so whatever zuck says goes and if other shareholders don't like it they can kick rocks. This is pretty much the same situation that people imagine is the case with privately traded companies, even though facebook is obviously publicly traded.

Shouldn't it be worrying that companies are required to make consistent gains* for shareholders and investors? At some point, a company will naturally reach a market saturation point.

* ETA: I meant "growth" here, not profit

s/are/aren't/ required to make constant profit