OK, let's go back to your objectives:
Your objective is to get the best versatile rig for your money, that can record both thunderstorms, trains and live music.
You have $1800 or so to spend at the moment. Do you see yourself spending any more on mics in the near future?
In that case, by all means go ahead and grab the Nagra. You already made up your mind that it's the best spent $1800 on the market.
But if you don't see yourself saving up more cash for mics later on, you need to rethink your gear chain.
The question here is not whether the Nagra is a great recorder or not. I'm sure it is.
The question is: are you willing to tape thunderstorms, trains and live music with your C1000's and end up with sub-par recordings until that day when you've saved up enough money to upgrade to professional grade mics?
I'd say that in this thread alone, the users who responded have about 150-200 years of collective taper experience. Some of the guys here are even university professors on recording and acoustics. Others are microphone retailers like Purple Dog, and have access to the full range of mics from all brands that they represent. Many of us have downloaded hundreds, even thousands of shows on archive.org, comparing mics, pres and recorders. To say that we don't have enough "personal information" to offer you our views on how to achieve your objectives is just plain unfounded.
I researched the issue as best I could and found many folks who really like the C1000's. Why else would I have bought them?
The reason why many users end up buying C1000 and similar low-end mics is that they only have $200 to spend. Perhaps also because they didn't have the same very specific usage in mind that you do. As others mentioned, they are designed to be on-stage mics, making them much less suitable for long-distance recording and ambient stuff.
And maybe because AKG are better at marketing and has a global retail outreach aiming at the hobbyist mass market, while more specialized mic manufacturers (think Schoeps, Gefell, MBHO, Milab, Nevaton) don't?!
In truth, you are offering opinions that I did not ask for. I will find out for myself. It's the only way.
Definitely, let your ears decide. Go to archive.org, download some shows with multiple sources and compare a C1000 recording with $1500 mic recordings of the same show. Come back here, upload them and let's have a blind A/B listening test.
If I were to ask a hundred people which is their favorite mic, I would get 100 different answers.
That's because your hypothetical question is too open. If you rephrased it to "which is your favourite mic setup that can record both trains, thunder and music, and my budget is around $1000-1500?", concensus here would probaly narrow down your choice to maybe 5-10 models + caps.
So who has the best answers? I don't know and there is no way of finding out short of using the mics and making my judgment.
It's up to you if you bother to do the research in advance or not. Users here do this research, all the time. Again, let your ears decide. There are lot's of sources on archive and other sites where you can compare mics. You don't have to
buy and use your own gear to make judgments.
Do you think they are less important than the mics? I don't think so. They are both important. So, I say, buy good pre-amps. I'm going to do that.
Nobody is arguing against you buying gear with good preamps. This is not what the discussion is about. What we're all trying to say is that the best preamps in the world won't output a good recording if you feed them with mics that aren't suitable for the task.
What do you think you would get if you spent $1500 on a mic? Something better? Are you willing to spend that much? I'll bet not. Why, because you don't place a great value on the fine quality sound which the great mics are capable of. But I am willing to spend that much. Just not now. I'm not ready for that level of quality.
You're contradicting yourself here.
IIUC, you're willing to accept less-than-great sound quality on your recordings and stick to your C1000's for the time being, but at the same time won't consider sub-$1800 recorders because they're not professional enough. Why? Wasn't your objective to get the best overall quality given your current budget?