Thomas Shey
Legend
This is a separate question from “who decides” or “whose hands is the process in”.
The group and GM enforce the rules regardless of what rules we are discussing. The rules decide when you fail a death saving throw in 5e, and the DM enforces those rules if there is an issue with the rules at the table.
That doesn’t make it any less up to the player to decide when the the PC is engaging in a given action.
Like…it’s literally an action the PC can undertake. It’s “I’m going to use my week of downtime to take a training endeavor to train that fire kick technique I improvised during the adventure.” And then the GM operates the rules engine in the sense of asking for rolls, but success or failure isn’t even up to the GM, it’s up to the dice and the success ladder.
Its not the training part I'm arguing about; training is pretty easy to set up that way. Its the advance-by-use part that I'm arguing about. I've seen numerous games that use that sort of thing and all of them require someone to decide when a "use" is legitimate. I'm hard pressed to see how you're going to dodge that.