> We've launched twins before. The two Voyagers, for example. Two Vikings, for another.
Good point. Do we know the marginal cost of the second one? Why don't we send twins now?
> When we build different airplane models, quite a bit comes from other airplanes.
I expect we're already doing that with space probes, to the degree it's possible. I feel like some of your argument is the old 'these people are idiots and I know obvious ways to do it better'.
A good question is, why doesn't NASA do it that way? It would be interesting to hear the response of someone there.
I can think of reasons it might not be effective - e.g., the probe for a few months of Europa terrain observation might be very different than the 10-year solar radiation observer, which might be much different than the Mars whatever, etc.
> I debated those experts here, and a lot of their arguments didn't hold up. For example: / 1. only need to [____] once
Those are the benefits of standardization and mass production and can be quite valuable, but not everything can be done that way. That's why we have different planes, cars, etc. I might want everyone in our company to use the same laptop, but different people have different needs.
I can come up with other possible issues, but I have no idea without someone with actual experience:
There can be a significant cost to engineering for standardization. Parts, assembly, a supply chain, a production line - maybe not worth the cost at the quantities needed. And there are things like the F-35, intended to save money by meeting the mission requirements of militaries in a ~dozen countries and services, including all of the US Air Force, Navy, and Marines - all simultaneously! Anyone who has designed even small systems would, I think, feel alarm at reading that. It took a little time and money to find a way to do all that effectively, please all those bosses, with an all-in-one tool. (Off the top of my head, just think of power supplies that can suit every possible demand on space probes!)
Weight and size are especially an issue for space probes, which would seem to make an all-in-one tool more challenging. The proposed solution often is modularization - a standard framework with interchangeable mission modules. In my limited experience trying it, it's a mess: when things break or need lots of extra attention, IME as a general rule it's most often at the interfaces between systems and between subsytems; the interchangeable module approach is asking for trouble. I know the US Navy tried that with littoral combat ships, and it failed. (Which doesn't mean it never succeeds.)
> What's more important? Solar system exploration or half-brain cats?
You really set yourself up ...