There are several advantages to Darwinian Difficulty. Let's return to the concept of "Subjective Difficulty". Because the majority of the mechanics are available from the start, as the player improves their skills, the game should become easier the longer they play.
The reason is that all the enemies, bosses, and situations are based around the core mechanics. The only changes that happen in the game are the encounters the player will face.
By learning the core mechanics of the game, players will learn to adapt to the new challenges the designers throw at them. Sections that gave the player a lot of trouble at the start of the game should no longer challenge the player once their skill has improved.
Developing the game using Darwinian Difficulty gives the designers a greater understanding of the skill level of the players, allowing them more freedom with their design. In a normal action game, the designer can have a hard time determining how good the players are at specific points in the game.
One player may have mastered all the combos by the halfway point; someone else may still be button mashing. With games designed with a Darwinian view, the designers know that anyone who got past the first level knows all the mechanics at hand, and can design around that.
The other advantage is motivating the player. Overcoming hardships or challenges brings a sense of a satisfaction. That sense of satisfaction can be felt by gamers who got through a game by their own skill. That satisfaction is a great motivator, as it is intrinsic to the player.
Replayability is another plus, because of the high degree of skill required; coming back to a game with Darwinian Difficulty after a long time can be a fresh experience. Similar to how if someone lifts weights, then stops for a long time, the muscles will atrophy, requiring the person to start small again and work their way up. This can also happen with games based on Darwinian Difficulty. Someone who beats the game, then stops playing for several months, will come back to find that they'll have to relearn those skills again.
With all this said, however, there are a few problems with Darwinian Difficulty that the designer has to keep in mind. First, is having such a high degree of difficulty early on hurts the appeal of the game. Games these days are being designed around having as much appeal as possible, which in this case means having a lower difficulty. A game where the player can die at any moment is not something a lot of people enjoy. Demon's Souls and its sequel, Dark Souls, may have created a lot of buzz, but they aren't truly mainstream hits.
Second, requiring the player to improve their skill is a great motivator, but there is a risk involved. The concept of "Subjective Difficulty" is just that: subjective. A challenge that one player breezes through without any effort can be a brick wall for another, stopping all progress. Losing a game because of issues with the design is easy to take. Losing a game because the player themselves was not good enough is a harder pill to swallow.
Getting completely stuck in a skill-based game is easier than in an RPG, due to gameplay abstraction. With an RPG, the player has other options of getting out of a tough spot, such as grinding levels or buying better gear. In a skill-based game, however, those options are not available, forcing the player to keep trying until they get past the section.
In this scenario, it's quite possible that that the player will get discouraged and stop playing. Stopping because of a brick wall will leave a bad taste in the player's mouth that will discourage them from playing again -- or outright quit the game -- which can leave a negative impression for future games from the developer.
Darwinian Difficulty is a hard mechanic to design around, requiring the designers to put in extra time fine-tuning the gameplay. The design is less about creating an imbalanced experience, forcing the player to climb out of a proverbial pit, and more about giving the player all the tools they need to succeed. Whether or not they'll be able to use them, however, is the big question. When designers successfully pull off the balance of difficulty, players will get an amazing experience few games these days deliver.