I've never done this, simply because I've never got 2500+ diamonds at any point but I'm sure if I had, I'd have made this mistake and it would really peeve me off. The only reason they have for not changing must be the fact they don't want to. They'll know mistakes will be made and people will just buy more diamonds to replace them.
I have hit max level when levelling a Toon by mistake though, and that's really frustrating wasting sooooo many potions on a Toon you would never max level. Getting Rot beard to 125, when I was just getting him to 60 to equip runes nearly made me throw my phone at the wall.
Yet you get "are you sure" prompts in other places and it constantly tells you that you don't have a full squad when using one Toon in Crypt. So they're OK with adding a failsafe button for irrelevant things, when it comes to something that'll make them some pennies they neglect it.