Two months without a commit could still be quite active and useful software, especially for a personal project. Where would you draw the line?
The graph on the chart I shared suggests that the peak of contributions was a couple of years ago, with occasional changes since then. This doesn’t make much sense to me, as the rendering quality looks great (at least in the videos—I’ll try the software a bit later), and it’s head and shoulders above what the scientific community is currently using.
Based on my experience (both personal and from colleagues), when a project is not in active development, the team starts losing knowledge of the codebase along with its context. For example, something that was at your fingertips while actively working on the project would be much more difficult to recall after a year. The difficulty of maintaining or extending the project grows over time if it is not actively worked on.
‘Stalled’ = contributions become less and less frequent.
If a project has stalled, there isn’t much new happening. For a simulation like this, the sky is the limit—you can make it as accurate as possible (e.g., accounting for light pressure - esp. significant around blackhole acceleration disk, the Yarkovsky effect, etc.)
The first is that different types of rendering have different uses; typically in scientific visualization this is broken down into essentially "viz for self, viz for peers, viz for others" and oftentimes the most well-used rendering engines are targeted squarely at the first and second categories. The visual language in those categories is qualitatively different than that used for more "outward facing" renderings.
The second reason is that I disagree with your assertion about the quality of the visualization techniques in use within science. There are some truly spectacular visualization engines for cosmology and galaxy formation -- just to pick two examples off the top of my head, the work done by Ralf Kaehler or that by Dylan Nelson. (There are many really good examples, however, and I feel guilty not mentioning more.)
As I said in another, rather terse and unelaborated comment, though, this is really, really impressive work. I think it's important that in praising it, however, we don't discount the work that's been done elsewhere. This need not be zero-sum.
Regarding the work of Ralf Kaehler: I have seen his renderings and looked through his articles, but to the best of my knowledge, no source code is publicly available. I don’t consider it fair to count it as something actively used in the field, beyond his lab and affiliated projects.
Disclaimer: that doesn't mean that there are no others, but their availability to researchers is limited to be widely spread.