Well i partly agree, and if i would be the one building the counterpart, i prolly had used presigned s3 urls also.
In this specific case im getting oldschool file upload request from software that was partly written before the 2000s - noones gonne adjust anything any more.
And ye, just accepting giant size uploads is far from good in terms of "Security" like DoS - but ye we talking about stupidly somewhere between 100 and 300mb CSV files (called them "huge" because in terms of product data 200-300mb text include quite alot) - not great but well we try to satisfy our customers needs.
But ye like all the other points - everything is solvable somehow - just needs us to spend more time to solve something that technickly wasn't a real problem in first place.
Edit: Another funny example. In a similar process on another provider i downloaded files in a similar size range from S3 to parse them - which died again and again. After contacting the hoster, because their logs litearlly just stopped no error tracing nothing) they told me that basically their setup only allows for 10mb local storing - and the default (in this case aws s3 adapter for PHP) always downloads it even if you tell it to "stream". So i build a solution that used HTTP ranged requests to "fake stream" the file into memory in smaller chunks so i could process it afterwards without completely download it. Just another example of : yes its solvable, but annoying.