"At its core, Star Trek was all about optimism. Were there non-optimistic themes ever presented in Classic Star Trek? Sure. But the overarching theme was optimism."

In part, violating this "rule" was what killed off enterprise, since the third season completely lacked this, in combination with the characters making quite unethical choices.

(Something else typically star trek: the main characters act ethically, and do a great deal of justifying any shady behaviour to themselves, this went out the window in the third season of enterprise, and is a lot less present in discovery as well.

The 4th season of enterprise returns to better quality, but at that time the damage had been done.