A completionist wants to have it all. All sidekicks. All avatars. All items in the MD. Choice is evil, let us have it all.
I believe the game has, over all, quite enjoyed from these tendencies of myself and other completionists. I think tending for people like us is in the interest of the game.
While the game is fairly friendly to completionists, it is not a haven. Just ask new players how they feel about the alpha and beta badges, or the baseball rewards, or the anniversary gifts they can't get. Ask dedicated players who disappeared for a while and missed the mining event. Those are very minor things precisely because I don't like to cut people out of too many opportunities, but they do exist, and we'll probably have at least a couple of them every year.
I tend to be a bit of a completionist myself, so I do understand. The part that's getting me here is where I'd want to complete an avatar collection for avatars, I wouldn't find myself inclined to also want to complete the avatar-as-item for the item collection. Not saying you're wrong for wanting it, just that it's hard to wrap my head around.
I'd also like to point out that this is inconsistent. Spending 20 or 30 mettle in vain is nothing compared to spending 200 IOU in vain. Has there been many bug reports about people buying a second precompletion training certificate, barcoded picture frame or personal copy of the internet? These are in the game for quite some time. If there were so many reports were they so difficult to handle?
Yes. There have
been comments and bug reports, because players have done this. Not many, but a few. They were
difficult for me to handle, in that I had to say "sorry, you're screwed, you've wasted those IOUs" to the regretful player. Not time consuming, no, but it instilled in me a desire not to create more of these situations, and it's almost directly a reason why I programmed the limits into my next specialty store for retcon.
Call it inconsistent if you like, but I call it "learning from my mistakes."
Let this protection from silliness be a matter of choice.
Add a general preference to the account setting which by defaults prevents one from buying silly things. Even call it that way "Protection from silly purchases". This will prevent you from buying not only the irrelevant mettle items but also the IOU ones and maybe other in the future. To be able to buy as many as you want, someone will have to deliberately remove this protection, and thus forfeit the right to complain about their silly purchase.
The opportunity cost here causes this to be an unlikely solution. It'd be a fair bit of extra time and potential for a lot of bugs coming out of the complicated additions for the protections and the toggle. I'd rather just turn off the protections and deal with bug reports than spend the hours trying to code this instead of new content.