I've read your question a couple of times and I think I see the gist of what you're going for, but if I'm way off, please let me know.
Pulling 2 sentences:
. . . but claims made that other planets have geology that is
extremely hostile to colonization (or even, in the case of Venus,
. . . .
Judgements like this seem very premature given our lack of hard data,
especially that of direct measurements of rock strata.
It sounds like your asking why scientists are pesimistic, especially in regards to colonization of other planets and the 2nd part, why do scientists jump to conclusions like "Mars has a solid core" - and I remember when that was the going hypothesis.
My answer is that pessimism is useful, especially with space travel, colonization or billion dollar missions. Nobody wants another hubble that takes blurry images, or worse, astronauts dying.
Both the dreamers, who want to expand into space and do everything we can, and the pessimists who tell us why this or that won't work are useful parts of the design and planning of missions. Space travel is crazy expensive and when people are involved, often very dangerous and it's good to have people to think of reasons why a mission might fail. I realize I'm off your subject a little bit, but pessimism has a place in any kind of big-budget planning.
Onto the 2nd and I think, more interesting part, why do scientists say things like Mars has a solid core, when in 2003 this was found to be incorrect.
First, not all scientists were saying it. Probably most scientists were in the "we don't know" camp. But "We don't know" doesn't make as good an article, so the scientist or team of scientists who says "Mars core froze solid, that's why it lost it's magnetic field", is more likely to get quoted than the larger number of scientists who say "We don't know why Mars lost it's magnetic field". - and I'm old enough to remember the "Mars has a solid core" articles - it was what was presented to the public, or at least, based on the articles I read. My mind is telling me that I read that more recently than 2003, but my mind is unreliable about such things. Google tells me that Mars' core is liquid.
Now . . . liquid is a bit of a lie as well, it's more like the consistency of pitch, but it's liquid enough to flow a little, or, not frozen solid and rigid (rigid objects that large still bend due to tidal forces), but rigid doesn't flow though it might rotate, and I'm getting a little sidetracked. Point is, these measurements are subtle, but the going theory is that Mars has a liquid core.
What you're missing is the scientific method and there's lots of articles about this, easily searched. I think a lot of them make it a little more complicated than they need to, so I'm going to summarize.
- Develop a working hypothesis (example, Mars - small planet, it's core froze solid)
- Develop a test for the hypothesis, which ideally provide evidence in favor of, or in disagreement with the hypothesis. It's worth noting that a GOOD hypothesis, often refers to whether it's testable, not whether the claim is correct.
- Publish your results - let others review them, repeat the test, reach a general consensus or disagree.
The heart of the scientific method is the testability part and the part where other scientists check the work. Not to sound like a Borg, but to the scientific method, belief is irrelevant, assumption is irrelevant, only the results and those who are trusted to review the results matter. The results should dictate the answer.
So, when a scientist or a team of scientist said at some point "Mars has a solid core, that's why it lost it's magnetic field" - that was a claim, which is little more than a logically constructed argument/educated guess. It wasn't an evidence backed study. Scientist know the difference between a claim and an evidence backed study because of their training and they are completely different things, kind of like how a lawyer recognizes the difference between an emotional or ethical argument and a legal argument. But people who read science articles may not see the difference, and so when a claim is proved false (like the Mars has a solid core claim) - oh my, they were wrong.
The funny thing about science is that it's OK to be wrong. They're not picking stocks or playing chess where making the right move is what matters. Science is about discovery and it's the testable hypothesis that moves science forward. Being wrong is OK, in fact, scientists have won the Nobel prize for being wrong.
The discovery of dark energy, basically everybody was wrong on that prediction, so much so that the data had to be double and triple checked before it was believed. The discovery of the cosmic background radiation - they thought it was interference from bird droppings at first and spent time cleaning up their radar dishes before they went back to look for whatever it was they were looking for. It was what they found that they weren't looking for that was the big discovery.
Bell's theorum is a fun one. Bell believed he'd found a way to disprove spooky action at a distance and when his experiment was finally put together, it wound up proving what he believed he would disprove (and proof isn't quite the right word, it's more a math word than quantum physics, but I think you get the gist).
Point is, it's the testable hypothesis that moves science forward and sometimes wins the Nobel prize. Not the idea and not even being right, but what can be demonstrated, that is, proved right or proved wrong.
Claims, or logically constructed arguments have a place in science. This happened because of that, and then "oh look, we were wrong". To a scientist, this is part of the scientific process and anyone who reads an article, or who goes back to the original research can usually work out the difference between a claim and an evidence backed study. Not everyone who writes articles makes that difference clear and many people who read the articles don't even know there is a difference. Hence, it's not always a good idea for scientists to talk to the press. Things can be misinterpreted.
I happen to think that a handful of your examples are true, but to the scientific method, it doesn't matter what I believe, it matters what can be proved - like that Tom Cruise movie. Scientists are actually pretty good sports most of the time about being wrong, because when a claim is proved wrong it opens the door to new discovery.
This isn't because science is sloppy. It's because the scientific method permits inquiry and logical prediction, such as "We think this because of that" and if what they think is later proved incorrect, that teaches the scientist something. In fact to the scientist, it's kinda cool when things don't turn out as expected. The public, however, can be less forgiving.
That's my take, anyway, hope that helps.