A few TB of space. If you have that full, how do you back it up? You don't have other slots on a MB or adapter with more NVMe ports do you? So if you have a few TB of data on NVMe, the only rational way to back it up is with SATA. If you want to back up that few TB of NVMe on SATA SSD, and want to do it with a single drive, that's a few hundred dollars. If you want to back it up with a mechanical drive, it's less than $100 USD.
You're throwing away a lot of money, and most people aren't willing to do that. There's no good reason to have data on an NVMe unless you have very little data. There IS good reason to use NVMe as a scratch area to build files onto with whatever work you do because a single NVMe can keep up with pretty much any app that's building that file. But if you have a few TB of data, NVMe becomes a very expensive solution. So I can look at my situation and see the problems that causes. I have 1 NVMe with Ubuntu MATE installed on it. I have 2 NVMe drives that are in RAID 0 with Win10 on it. The Win10 NVMe drives are 1TB each. I have about 1TB of GAMES installed. I have some data on that NVMe RAID, but I have TBs of data. I'm already up to about $400 of NVMe and would have to add an NVMe adaptor if I want more storage for data. Or, I would have had to spend an outrageous amount of money for 2TB NVMe drives so I would have 4TB RAID 0 config.
There are plenty of people who have too much data to even begin to consider NVMe for that. Imagine 10TB+
So, the common sense thing for people with a lot of data is to load the OS, apps and some data onto a system NVMe drive. If they do work with video or large media files, or they do other types of work where they need 100s of GB of work space, then a 2nd NVMe drive will be used for that. That's the limit right there for most systems. Some people, like me have a MB with 3 NVMe ports. For those people, most are STILL not going to able to use an NVMe drive simply for data. You have fast enough access to data files, including large video files with a single mechanical drive. If I want to compare pricing, I can buy a single Seagate EXOS X16 14TB drive where I can read data around 230MBs. It's fast enough for data access. I can now buy one for around $250 USD. If I want to try to put that on SSD, AND I have 10TB of data, I would need 5, 2TB drives which will cost me about $1000 USD. AND, I would need to buy a SATA port adaptor for many MBs where 4 ports are starting to become the normal config. I could go up to 4TB SATA SSD drives, but more often than not there's a price premium jumping up to that size, and data access is a little slower.
In other words, it's about the amount of data people have, and data access doesn't need to happen at NVMe speed, although when working with video files, it's nice to have an NVMe as a work space because it's fast to write to, and fast to pull the data from to permanent storage.