Can you give me a theoretical example? Seems like saying, "Suppose I have a legitimate case for needing 45 trips to the buffet line."
I get your points, but I think outliers are outliers, and correspondingly they should be thinking as such. So if I had, for whatever reason, a need to store 17TB of data, I would need to be as clever about how I stored it as I obviously was clever enough to even possess 17TB.
I can give you a practical example. I do a lot of photography/videography in my spare time (not professional, have never been paid for it). I shoot RAW photos on a Sony A7RII. Every photo is ~85MB and I can shoot 700 photos in a day. Assuming I shoot one day every weekend, that's 3TB of data a year (that's actually a close estimate when I account for vacations and such too). Then there's video. I own a Panasonic GH5 and record on an Atomos Ninja Inferno, which outputs 4K 10-bit ProRes video at 60fps. This consumes ~1TB per hour and I tend to store it for archival purposes since it only costs ~$25 to do so for pretty much ever.
For a less extreme case, one of my old school teachers has been doing photography as a hobby for ~15 years. She's amassed ~10TB of photos over that time and currently stores them on piles of HDDs and a few spindles of DVDs scattered around her house.
And yes, I absolutely accept that I'm an outlier but any service that offers an unlimited plan is explicitly advertising itself to outliers. They're not putting themselves out there as someone with a 1TB, 2TB, 10TB or even "enough for 99.9% of people!", they're saying that they're giving storage with zero limits.
Your buffet comparison by the way isn't a good one. A buffet isn't truly unlimited, it has a time restriction, a condition that you can only eat what you take, you can't leave with it and the fact that a human being can only physically eat so much. They haven't defined a limit but there certainly is one.
When I was younger I worked at a recording studio. Each song consumed about 10GB, and it was convenient to return to the sources at a future date once the mix was delivered to the artist. Each artist recorded about 10 songs, and in a year we could do 50 artists.
I can't imagine what animation and video production houses go through per day. Having a backup is completely necessary for these folks. I could go on, but I feel 17TB is pretty common for businesses, so I'm not sure why you're asking the question. These days you can get 17TB for only $700.
So either the guy has a legitimate business reason (in which case he's definitely chosen the wrong service (level), or he's not using it for business.
And I don't think they're at fault like you're suggesting. Suppose I have a legitimate use case for storing 17TB of data and need to do it on a budget. During my shopping, I discover ten $1000/yr services for 20TB storage and three $100/yr services for unlimited storage. To me, the $100 services seem like scams or that they have no idea what they're doing. Amazon, Backblaze, and Dreamhost are literally giving off these vibes! Regardless, I'm on a budget so I purchase Backblaze and upload my 17TB. But in the back of my mind I'm nervous that they will shut off my service, despite the contract I entered with them to keep an "unlimited" amount of data for the price I paid. Or, if the contract says they have the right to terminate my service if they deem the amount to be too much, that would make nervous even as a normal user of the service, since the definition of "ridiculous" might be decided by some judge in a court if the service decides to cancel the contract and my company decides to sue. Would you really want a judge to decide that, or would you prefer a hard limit to be placed on the website and the contract in the first place? I truly don't understand the appeal of "unlimited" plans compared to simple hard limited, tiered plans.