How can that claim be off the mark? It's entirely true.
And I don't understand why a player would necessarily do that. A player might do that, but he needn't.
A person can cover their ears and close their eyes while insisting that the external world doesn't exist. It doesn't make the practice any less idiotic.
It doesn't follow with necessity, it follows with probability. It's the same basic concept behind why developers normally have a tab strictly to keep track of your main quests. It says to the player "hey, this is important". If high percentage of players are capable of recognizing gaming tropes, it's a pretty damn good assumption to follow.
Why would we assume that?
Because you made a claim, specifically with regards to the overarching plot? In which case, wandering off the beaten path is irrelevant.
And, as I pointed out, there are other trails to find. And the trail you insist everyone can see can be entirely missed, not just by not drawing the same conclusions as you, but by encountering a different set of evidence.
So you mean that every player isn't exposed to Gorion's speech about reaching the Friendly Arm? That if they proceed there, Khalid and Jaheira won't recommend visiting the Nashkel mines? That if the player follows that tip that upon reaching Nashkel they won't immediately reach Chapter Two, receive strange dreams/quest journals telling them that they're on the right track?
What's that they say about leading a horse to water?
When I played Oblivion, it never crossed my mind that I should follow the King's instruction to seek out the Captain of his guard. My character had been locked in a dungeon for reasons unknown to me, the King refused to tell me when I asked, and escape was now possible. Actually looking for an agent of law enforcement right then struck me (the player) as most likely a trap, rather than a quest hook.
That most people draw the same conclusion doesn't make that conclusion correct. It makes that conclusion common, and nothing more.
All irrelevant. You made an in character choice. There is a difference between your character, lacking metagame knowledge, being unaware of the main plot and you, the player, being unaware of the main plot. Your initial post made reference to the latter, not the former.
Even if, in game, it was a trap, from a metagame perspective (as the player) you could quite easily figure out that you would need to progress to the Captain of the Guard to continue that particular plot thread.
Baldur's Gate is just as straightforward linking the player to the various locations they need to visit as Oblivion is.