Why don't we just go all the way back to the beginning and just keep flinging poo at each other?
You think what was done in the past was a good thing? Ignore the outcome of all of it for a second, before you answer that question. What it boils down to is simply "Would you do it today?" Force your religion onto a native tribe. Relocate them. Force them to live your way. Kill them because you want the ressources they are sitting on. All that stuff. Would you do it?
You can't erase what is done and no one said growing up is easy. You also can't view the world of hundreds and thousands of years ago through the lens of today's moral parameters.
So the Prime Directive isn't a good thing... because you'd obviously go back and change things you felt weren't right.
Hence the temporal prime directive, put in place in the 24th century...already unofficially part of most logic within TOS: "Tomorrow is Yesterday", "City on the Edge..", "Assignment Earth".
No one wants to change that past but cultures do grow and adapt over time...so its time to apply what we've learned...