If this could be done it seems like the ideal compromise. Everyone gets what they want.
That said eventually more modern languages will be dependencies of the tools one way or another (and they should). So probably Debian as a whole should come to a consensus on how that should happen, so it can happen in some sort of standard and fair fashion.
Does it seem that way? It happened at least once (but could have happened many times without "taking over"), and certainly one sort of life seemed to successfully out-compete all others. But none of that says single-origin to me.
Early on I would expect a whole lot of "horizontal gene transfer" sort of things to have taken place. So for example in addition to actual horizontal gene transfer, there are mechanisms like one organism enveloping another to eventually become organelles, co-opting products from each other, etc. All of which would act to homogenize life and make certain process ubiquitous.
Finally, there's an outside chance that "there's only one way to do it".
You are right: this is indeed no smoking gun (and it isn't hyped to be one). This is more like "we can't rule out life having created this, but there are alternate explanations which have also not been ruled out".
Unfortunately most of the evidence is going to be like this. The chances for better evidence would probably require a sample return of some sort, and even then I wouldn't expect a smoking gun (either way).
I think the building blocks of life are so common in the universe it might be a case of "easy come, easy go". It wouldn't be surprising if simple life happened anywhere it was given half a chance at all, but one would equally expect that it would die out just as quickly when conditions changed (which they certainly did on Mars).
And of course nothing is ruling out life in the nooks and crannies of Mars.