One house rule I apply to replace any "bleeding out from negative hit points" rules in whatever system I may run is to to have dying be a pure players option (or GM option for NPCs) when a character is reduced to negative hit points in the still-alive-but-bleeding-out range. The player may choose whether his character (a) dies at once, (b) lingers for a lesser or greater length of time before dying, or (c) clings to life long enough to be healed. All without reference to any "staunch the bleeding" actions or die rolls being required to keep the character alive - my house rule drops those.You've articulated my problem with the eschewed social contract far better than I could! This was mostly the reason I put "assuming that the PCs are competent" in my wishlist in the OP. I like games that admit that the PCs are the main characters in one way or another. This does not mean that the PCs will have plot armour or that they will always succeed, but their failure should also be narratively meaningful (dying "like Boromir", as you say).
Are there any specific retroclones or house rules that you think really help reinforcing this social contract?
(b) allows the classic bits of a final dying speech, or of a mysterious NPC stumbling into the tavern, whispering a few mysterious words and/or handing the PCs a trinket, and then dropping dead of injuries. (c) means letting a character live for an absurdly long period at negative hit points - not just the time needed to get healing spells cast, but (if necessary) the days or weeks needed for natural healing to bring the character back up to positive and full hit points.