> my primary use case is to show someone something. No one will come back to that file ever again.
Well, the whole point of this discussion is that (unless if you do it by PM) this counts as (folk)lore, so you shouldn't assume that.
It's simply the experience and some of the tech stack from VK because most people who built Telegram worked at VK before that. VK, being a Facebook-like social media service, allowed people to upload all kinds of things — photos, videos, music, and even arbitrary files up to 200 MB. Telegram even praises itself on "unlimited cloud storage". I never asked anyone about it, but I'm sure that it works out to each user, on average, not consuming much storage, because most of the media content people send is compressed images.
I'd suggest uguu.se for this. Hosts up to 134MB per file for 48 hours. No BS web interface and can alternatively use local ShareX screenshotting tool to upload directly to it to grab a link automatically.
Discord will render still the thumbnail for the link and then after the file expires it will just stop rendering in the chat history.
There are a few free, donation-supported temporary file hosts of a similar nature.
Direct file transfers don't incur any storage costs whatsoever, it's only bandwidth. They also already have the relay servers to support WebRTC calls. They could've solved this problem for little to no additional cost if they wanted to. The problem is that they don't want to.
As for paying, Telegram has mostly the same business model (selling subscriptions but also ads in channels for free users) and allows uploading 1.5 GB files for free. With the premium subscription that is doubled to 3 GB.