Unstable things tend to wobble around until they find a stable configuration, and then remain there (by definition). This goes for pretty much everything.

Introducing some external perturbation can destabilize the thing until it eventually settles into a new stable configuration. But if you've built up lots of systems around the old stable configuration this kind of sucks.

(Also, stable configurations can be hard to reach, and "eventually" might be a rather long time.)

"Why was everything always good, peaceful and stable in "indigenous environments" until "the colonizers " show up?"

One thing that may help resolve this in your head is that invasive species taking over is the exception, not the rule. The vast majority of the time, when a species is taken out of its native environment and dropped somewhere else, for whatever reason, it dies. Maybe immediately, maybe a few generations get off before the population drops to zero, maybe they do great until they're slugged by winter or the rainy season, most of the time the "home team" will kill the visitor without so much as metaphorically noticing.

However every once in a while the stars align and the new species fits into a slot the home team didn't "realize" existed, or they hammer a weakness that the rest of the ecosystem had just been coevolving around for a long time, and you get the invasive species. It would feel like winning the lottery, except that the invasive species then get to grow exponentially and loom very large in our minds and our experiences. They are, despite that, the rare exceptions and not the rule. The rule is that a species dropped into another full ecosystem with no coevolved slot for them just dies.

Take a look at "1491: New Revelations of the Americas Before Columbus" by Charles C. Mann: https://www.amazon.com/1491-Revelations-Americas-Before-Colu...

Mann attempts to reconstruct what the Americas were like before European contact. More importantly, he makes a case that some American Indians had a higher standard of living than Europeans.

More importantly, everything really was mostly "good, peaceful and stable ... until the colonizers" showed up. The disrupting factor were the pandemics that happened; not that one culture was superior than the other.

The ecosystem adapts into a semi-stable equilibrium that’s disrupted by the introduction of foreigners having different characteristics.

Probably because after turmoil from climate change due to melting glaciers after the last ice age things more or less stabilized after thousands of years. The current state will also stabilize but will take some time. I guess people like the status quo -whatever that is.