We human beings may be near the end of Darwinian evolution โ no longer required to become the fittest to survive
This suggests a common misunderstanding of what "fitness" means in an evolutionary sense. It doesn't mean being athletic, or healthy, or bigger, or any other specific trait like that. It just means that your progeny are more likely to be a bigger portion of future populations than other "less fit" organisms.
The only way that "survival of the fittest" were to go away is if every individual had exactly equal chances of contributing to the next generation's gene pool. That's not going to happen under any plausible scenario.
And machine life is also subject to evolution, so there's no escaping it there either. What exact characteristics constitute "fitness" may change, but there's still going to be something and it's probably going to be comprehensible to us since the laws of physics are the same for everyone.
Perhaps they'd have new ways of getting energy that we just can't envisage yet.
Maybe. But those new ways of getting energy don't make the old ways cease to exist. Artificial life could be huddled around black holes drawing on their stored rotational energy, for example, but when all the nice juicy locations in the local black hole's ergosphere are filled then the "poors" still have the option of falling back to building Dyson swarms. Why wouldn't they? If you wish to explain the absence of Dyson swarms you need to come up with reasons why Dyson swarms wouldn't be built at all, not just come up with alternatives. Life expands to fill every niche it can get its hooks into.
We have evolved through Darwinian pressures to be an expansionist species.
Sure. And as a result, our non-expansionist cousins became evolutionarily irrelevant. The same thing would happen with machine life. If you've got two von Neumann machines, one of which is programmed to be "content" with limiting its future descendants to inhabiting just a single planet or single solar system and the other of which has no such limitation, then in a million years it's pretty obvious which one will be present in vastly greater quantities. Switching to a less-expansionist strategy only makes sense when there are no longer any accessible places to expand to.