I agree with most of the rest of your post, but I don't think this is true. Nobles can be the primary focus of your deck a lot of the time. You don't want to use them as a village (trying to use Nobles as a village IS a beginner trap), but they function as ably as most other sources of +cards. And the extra VPs can make up for the slowness of building the engine. You do of course need some way of being able to afford the Nobles, but THREE Golds is way overdoing it. Usually a couple Silvers and money from actions is good enough.
I don't think we're disagreeing all that much. When I say linchpin, I mean that I've seen green players think that Nobles is sufficient as a primary source of Draw and Actions simultaneously, which we agree will always end in tears. Nobles can be a significant engine component, but not the
only engine component, especially at its price point. Three golds may be overdoing it, but I've won more games than I've lost by being patient and building my deck, while my opponent rushes for Nobles and buys the first two.
----
Switching topics, I see rookies mis-play Trash for Benefit cards with regularity. Inexperienced players are often uncomfortable sacrificing decent cards to a TfB. This severely limits their ability to make the best use of what these cards have to offer.
Part of this, especially with Remodel, is seeing the word "Trash" and thinking the card is a trasher, and using it mostly on Copper and Estates. Trashing a single copper for no or small benefit is not effective use of a card from your hand, and is usually a waste of a turn. Remodel is better used to gain Golds than trash Coppers, and I've been pleased more often than disappointed when I cross my fingers and draw 5-6 cards with an Apprentice. Newbies don't make those big plays, or even mid-range plays, often enough to make those cards worthwhile.
By contrast, Upgrade is pretty efficient working on your starting deck, so it's harder to mis-use.