/「IT界のサグラダファミリア」みずほのシステムついに「完成」 「スタッフロールとか出して賞賛したい」 : J-CASTニュース https://www.j-cast.com/2019/07/16362688.html?p=all
The reason Mastodon doesn't keep original uploaded files but rather converts them to more space-efficient formats/dimensions is that storage is not cheap for the average server owner, and we try to reduce the pressure on them as much as possible.
We could think about keeping high-fidelity originals if an upload quota feature was added (configurable max. disk space per user). This is non-binding poll to see if that's a direction worth pursuing.