What makes you so sure that within 300 years, the Mars colony won't become an existential threat with interplanetary nuclear weapons pointed our way? They may even take to calling themselves "Martians" as generations live and die on Mars with little connection to Earth. Like any civilization they'll have their own language, culture, and values.

The British colonized America in 1607 and 170 years later those colonists were calling themselves "Americans" and waged war for independence. What's it going to be like when that happens on an interplanetary scale?

That's surely a risk. But even in a worst case scenario the chances of a humanity ending scenario are certainly reduced by having more satellites of human civilization.

Could become super handy if aggressive aliens happen to arrive. ;-)