Inspiration Dice in D&D 5e [duplicate]

I use a milestone progression so relative PC power levels are equal and differentiate by giving out inspiration to for good RP, creative problem solving, and heroic actions.

Is it balanced to allow PC’s to bank their earned inspiration dice to purchase upgrades to their characters for "X" number of Inspiration Dice? I’m thinking a skill/tool proficiency, feat, spell slot, etc..?