Pleeeeeease 
I want to use developer products because they are easier to facilitate their in game purchasing but I also would love to show them on the website and only allow their purchase once (Things that can be done with only game pass). My problems would be solved if MarketplaceService.ProcessReceipt was fired after the sale of game passes.
ProcessReceipt is designed specifically for devprods. The entire way it works - how it’s called and what it returns - is designed with devprods in mind. There’s basically a whole devprod API, and allowing that to mingle with other product types would just cause confusion.
Why is MarketplaceService.PromptPurchaseFinished not a solution? Rather than trying to expand the devprod API, which really has nothing to do with your problem, I think the proper solution would be to expand the gamepass API.