Evolution (why do you keep calling it “Darwinism”?) can explain a lot of different outcomes, that’s true. These outcomes are the result of different circumstances, which obviously exist in nature. These aren’t all “just-so stories” - we tend to like to test our hypotheses. We might say “I hypothesise that this group evolved this trait for X reason”, and then go out and try and test it. For example, if we observe a cave-dwelling creature without eyes, we can test whether this was the result of lack of selection for eyes or active selection against eyes. If we observe a cave-dwelling creature with eyes, we can test to see if those eyes are serving a subtle function that causes them to be selected for - we can look at genes involved with the developmental pathways that produces the eyes and see if they appear to still be under selection or not. But, sometimes the hypotheses can be hard to directly test, so sometimes we have to settle for “that seems plausible but we don’t really know”.
The ability to explain different observations is not a bad thing in and of itself, it comes down to the evidence for, the plausibility of, those explanations.
Sorry, but in what universe are Kodiak bears and Polar bears "living in the same wintery environment?
Google image search results for “Kodiak bear alaska”:
Google image search results for “polar bear”:
Notice anything different? About the background perhaps?
That’s a good example. Do you know why it’s possibly to work things out after they happened? Because there’s so much more data available! The more economists learn about the way the global economy functions, the better they can understand it, and in some cases they can see things coming (e.g. how some people saw the 2008 financial crisis coming).
Surely you realise that those are fundamentally different kinds of “predictions” that you’re requiring the biologist and the chemist to make. Predicting the properties of a hypothetical molecule is child’s play compared to trying to predict the state of an ecosystem decades or centuries from now. This largely comes down to exactly how much we know about chemistry versus biology, and especially about the particular scenarios. This isn’t really an “evolution” thing, it’s a “biology” thing. Biology is messy. Life is messy.
In order to predict whether the rabbit would drive the existing mammals from their niche and to extinction we’d need to know much more about the specific small mammals in question, as well as the rabbit. With a few years of study it might be possible to ascertain the fitness of the rabbits in that environment, relative to the other small mammals. It would be possible to work out the available food sources, habitable areas, etc. We’d have to study the larger predators and pathogens present in the environment to see how they might interact with the invasive rabbit species compared to the existing populations of small mammals. Only with all of this kind of information could we begin to feel comfortable making predictions about the state of the ecosystem in the future.
You say “natural selection has always avoided that challenge”, and your hypothetical is also mostly about that. Are you saying you’re skeptical that natural selection actually works?
There’s a lot more to scientific predictions than questions like “what will X be like in the future?” More often, they’re along the lines of “if we run X experiment, we’ll get Y result”, or “if we collect X dataset, we’ll see trend Y”. The theory of evolution is capable of making plenty of such predictions, and has lots of nice examples of confirmed ones, which you must be aware of.