I will give you props for trying, but I don't think that system would work. Thinking outside of the box, while appreciated, doesn't mean thinking backwards.
At the beginning of the game, many examples give you all your powers to test and play, but its a tease, a carrot on a stick to keep you going, since you know you eventually will get as strong as that, or even more. To revert that mechanism would discourage people to keep playing, since they know they will progress from Superman to Jimmy Olsen.
The idea of a hero's journey is that the main character grows during its adventure, learns more skills and became more confident. The player is not overwhelmed with 1.000 spells and its variations, instead he/she plays with them and decide which ones he likes or likes to try better as he uses them. Your idea might work on some stage (maybe depower the hero before his battle with the final boss, or the Bioshock level where you loose all your upgrades one by one), but as an entire game concept its hard to sell the idea of "you are like Luke Skywalker... you start as a jedi and end up as a farmer"
Besides, I hope its not meant to handle the difficulty automatically. For starters, that is far a temptation for developers to keep using the same enemies... after all, a rat would be a little nuance at the beginning, but a veritable challenge near the end. For you to create bigger challenges, you don't need to think of harder obstacles, just throw the same obstacle you throw the last 10 hours and let the stats system work itself out.
Finally, the problem is that it restricts the options a player have to play the game. If you like to play as a stealthy, melee character, it might work on easier enemies (early in the game), but it won't work as you progress through the game. That means you can either experiment with the game, under the idea that you might have to replay large sections of the game if you get stuck in a place of the trimmed tree where you can't face the challenges ahead, or forget about playing the character you want, instead use a guide to see which character class is more useful and less likely to get stucked. If you put all the options on the player and a timer to lose them, you give them a sense of urgency and force them to play with guides, because they won't know how "less powerful" a weapon will became and how much that will that affect them.