Counter question: Does it matter (and I suppose if the answer is yes, It’d need a follow-up “Why do you believe that”)? From my perspective, if it matters how you acquire the rare item, then we’re no longer really talking about the rare items any more. We’re now talking about having achievements to show off.
Which reminds me (in a really roundabout way). Whatever happened to the Stickers concept that Boundless had a few years back? Unique stickers seems like an even better way to show off achievements than additional blocks.
I guess that could be a solution, but I’d have some reservations about it. If you can earn decent amount of coins that way, it will likely start an inflation problem. If it’s not worth enough coin then you still get rewards that are essentially worthless but for a pittance. If available for sale, these sort of things will find the right price on their own without further developer monitoring and potential intervention.