If realism is your immersion, the problem is both how XP is earned and how it is spent. A realistic progressions system should have production follow progression not progression follow production.
"I've decided that" takes what should be a dynamic system and makes it static. You know that route running is improved because you force it with spending XP on route running. I don't want to make "all that crap up myself", I want to be pleasantly surprised or disappointed with a players progression and what they progressed in. I want my influence to be realistic not god-like.
I'm glad you understand my desire for randomness and realism. I also understand your desire to completely control progression.