As one Creationist put it,
If these theories are valid, they would remain more stable, rather than fluctuating radically every few years.The scientist's answer is that this would be a good point, if the modern theory contradicted the old theories. But in fact it doesn't.
When Darwin published his book in 1859, he suggested that species are not fixed. He said that they can change, in time, shaped by mechanisms such as natural selection and sexual selection. He suggested that new species can come into existence. He said that all life on Earth was descended from a single common ancestor, or at most a few common ancestors. And he suggested that biogeography was convincing evidence.
Today, the theory still says exactly that. Each of Darwin's major points still stands. Well, we've eliminated the possibility that there was more than one common ancestor. And the genetic evidence turns out to be even better than the biogeography evidence.
So basically, in 142 years, all that's changed is the details.
Note, by the way, that scientists do not apologize for changing theories. Scientists specifically try not to be rigid or dogmatic, and they expect to change theories whenever new evidence is found. It's actually quite surprising that so very few of Darwin's ideas have had to be discarded.
Note also that it's inconsistent to accuse scientists of dogmatism, and also accuse them of changing their theory too much.