How is it like that? Where is the rule stated that players are not to build up skill over time, and play better as a result?
It's a recommended table rule from page 235 of the DMG.
On page 241 of the DMG it implies that some DMs will ignore inspiration because it adds metagaming.
And what does advantage mean here? What "advantage" is the player getting? Over whom?
You don't see what advantage the player is getting by being able to just have his PC "know" a weakness so that it can be exploited when the PC doesn't actually know about the weakness? And it's clear what the advantage is over.
Why should I take your pronouncements about D&D's rules and principles more seriously than his?
What pronouncement? I've said nothing about how YOU should run YOUR game. I leave that bit of arrogance to some others on this forum.
But suppose that someone decides to try and make decisions from their PC's point of view, who gets to decide what the PC does and doesn't know? As a player gains expertise, why can't they play their PCs as more and more expert and capable, should they wish to do so?
There's a difference between capable and knowledgeable. And by RAW it's the DM who decides. That's why there are rules for determining what the PC knows that leave it to the DM.
It's not how I generally play D&D. I generally allow players to author their own PC backstories, within common-sense limits. (And what I've suggested doesn't in my view exceed those limits.)
Of course. It's your prerogative to alter the rules however you wish. If you want to give the players the ability to author, you can. The default of the game, though, is that they can't except for a bit of character creation and advancement.
I don't understand what you mean by "players/PCs". Those are pretty different things - players are real people who are playing a game; PCs are imaginary people, elements in a shared fiction.
Combat challenges both. It's not purely a player challenge where the player can just ignore the rules and have his PC know everything about all monsters and use that knowledge to his advantage over the monsters.
But in any event, not all RPG play is about challenging the players, at least in the sense of testing their guts, their strategic acumen, etc. As you seem to acknowledge in one of the quotes above, play can be about pretending to be someone else. In which case the challenge to players is to faithfully portray that character. A player can meet that challenge by playing a PC who doesn't know about trolls, or by playing one who does. It doesn't seem to make much difference either way.
Sure. I'm talking about D&D combat, which is a challenge to both the player(strategically/tactically) and the PC. And I disagree with the bolded assessment. Trolls become MUCH easier when you know about their weakness. Demons become much easier when you know what they are resistant and immune to so you can avoid wasting or diminishing your attacks. It makes a significant difference to know vs. not know.
Just look at the following example.
A group of fighters who use non-magical greatswords with great weapon fighting in every combat come across a monster that is immune to non-magical weapons. They just decide to use their magical daggers, despite their PCs being totally unaware of the immunity. How is that not going to make a significant difference in how the combat goes? How is that meeting the challenge in the same way?