How species stay unchanged over aeons

The answer would seem to be that if a change doesn't dramatically increase survival then it has little effect. Not all changes are good ones. We already know that but many changes are probably just neutral and aren't good or bad.

It's probably hard for a cockroach to improve any more on it's ability to survive.
I take issue with this premise near the start of the article.
If evolution can happen so quickly, why have most species on Earth continued to appear the same for millions of years? (paragraph 3, emphasis added)

Most species? I’m the first to admit that my knowledge of evolutionary theory isn’t deep and stand to be corrected, but it is my understanding that nearly all extant species have changed over time. I thought the situation was the exact opposite: few species display the paradox of stasis, and the ones that do seem to be mostly some species of reptiles and fish.

Besides, either way I see these statements are problematic as the vast majority of all species that have ever existed are now extinct and have left no fossil record. So I can’t see how we can determine the extent to which the paradox of stasis actually exists as a problem.