I actually disagree with you that fantasy settings like the Elder Scrolls or Dungeon&Dragons are less historically accurate than ones that try to be realistic.
The simple fact of the matter is that settings that are pure fantasy have nothing to do with our history and therefore whether they bear any resemblance whatsoever to an historical time period in our world is entirely irrelevant. It's fiction, plain and simple.
On the other hand, a game (or book, or movie, or whatever) who does claim to at least partly represent part of our history (whether they're alternate history, historical fiction or an actual documentary) can be judged on its accuracy. Which is probably one of the reasons game developers don't use that kind of setting more often, since proper research takes time (and therefore money).
That's one perspective, and you are entitled to it of course, but you are right we very much disagree on this point.
Fine, let's just agree to disagree on that one.
And while I agree that there are time periods where historians are reduced to little more than conjecture when it comes to historical facts or how the people of said times lived, I don't think early 15th century is really one of those times, at least in Europe. There were plenty of people who kept records at that point, from monks to Italian bankers to actual historians, and Bohemia was an important enough country at the time that we should be able to have a good idea how people lived there.
There certainly are times where more and less records were created (and survived) but even in the times where there are many making certain that you've followed a thread which has solid backing and that such backing isn't distorted by contemporary bias (either theirs or ours) or outright misrepresentation (revisionist history has been around at least as long as Egyptian Pharaohs), is another thing again.
My dear friend, who is currently finishing his post graduate studies in history, has very firmly educated me during our semi-frequent discussions of history and culture, that making statements which are either too unequivocal or too confident is very likely to mean that you are making statements about history which are in some manner unreliable.
I don't claim to be an expert in the field by any means (I dabble here and there but am no professional) but when someone I trust who is a practitioner of the field tells me something I do tend to take them at their word.
That aside a lot of this seems to revolve once more around the our simple difference in perspective about the accuracy of a setting. To me a setting which is more like the historical context is more historically accurate than a pure fantasy setting which makes no effort to contain historical representation, but again from my perspective this is all on a spectrum there aren't really any binary answers to be found here (well, okay, something set in the future isn't historical... unless it's A long time ago in a galaxy far, far away.... then maybe ;) ).
While I agree that a historian should be cautious about historical sources, should strive not to let any kind of bias taint his interpretation of said sources and should be ready to admit when there just isn't enough material to know with any kind of accuracy what happened, he also shouldn't fall into the opposite extreme of doubting everything and claiming historical accuracy is impossible to achieve.
The fact is that if you have several contemporary sources which say the same thing, there's no reason to assume they're all lying unless you have some other documents that lead you to suspect they are.
To go back to the original point, the reason we know for a fact that plate armor was extremely expensive and rare in the early 15th century is that we still have plenty of records that show it was, from armorers' bills, to letters between noblemen, to bankers' records about loans they gave out.