“Storage management is expensive”
It's really not, though. Discord has 200,000,000 MAU. If every single one of them uploaded a file every month (of pretty much any size) and Discord tossed it into an AWS S3 IA bucket, it would cost them $500 to store that data. Their total S3 bill for storage would be five hundred US dollars. Storage is dirt cheap. AWS doesn't even charge per gigabyte on that storage type, it's so cheap; they charge for downloads.
So, ok. Let's talk downloads. If each of those files were 25GB and downloaded twice (probably an underestimate, but not everyone is uploading files, so I'm going to make the completely unfounded assumption that it'll all shake out), it would cost them a couple hundred thousand dollars. Which, ok, that's much more significant than $500. But Discord made $575 million last year—so the S3 download costs would be 0.03% of their total revenue. They probably spend 2-3 times more on coffee.
Storage management is emphatically not expensive.
My guess? They just saw that the higher upload limit was eating into their Nitro subscriptions.
I'll have to check my math again. But are people uploading more than that? On my friend server, with 50 people, we've had about a dozen uploads all year, and they're all pretty small PDFs and images. Everything else is rich links.