So why double up... on gold its already in story a valuable resource which you can gain followers or many other things. Why make it experience when it means not getting experience sometimes when the character really did accomplish a lot? Like that skill challenge which allowed them to save their ally I gave. In 1e if they figured out how to get him back the only experience they would get is???? In 4e they would be getting experience for overcoming the challenge (OK they did get a live ally if they succeed)
Because treasure is an objective value that can be used as a gauge for the value of the challenge faced. Treasure in classic D&D is allocated based on the threat that is expected to be faced. The deeper levels of the dungeon feature greater threats and greater rewards. The lair of a dragon will contain greater treasure the a hole with some goblins in it.
This allows players to tweak the risk/reward dial by deciding how much risk to face. Do they delve deeper and face more risk at the possibility of greater reward. Do they stay in the lower levels and play it safe, but recover paltry trinkets.
Treasure for XP is tangible and objective. If I find 2000 gold that is 2000 xp. It removes a lot of the issues with milestone style leveling where you have to guess how much saving the princess is worth. It also makes it really easy to figure out things like wealth by level... if my players encounter a 4th level baroness in her demanse, I can pretty easily figure out how what resources she has (men-at-arms, servants, arcanists, etc). The sum value of her domain is roughly equal to how much XP it takes to get to 4th level.
This makes it a lot easier to world-build on the fly if this comes up in the middle of the game.
I run adventure games. The focus of my D&D games are on exploration of the unknown places in the world.
There is a world and there are places to be explored and adventure to be had. In the example you provided... I would consider that a failed expedition. The party attempted to delve somewhere but got laid up by some threat and had to find a way to escape. They failed to recover treasure, so they wouldn't get xp. It happens... sometimes you go in and end up with nothing. It's part of the game.
It sounds like this is probably not your style and that is fine. I'm not expecting you to agree with me. This approach is probably not appropriate for a political intrigue game or such. I wouldn't use XP for gold if I were to run such a game.