To be fair those are VERY new and 10TB is probably an accurate estimate of the /average/ NAS user. I have 150TB (I am not average) and the max drive size I have is 8TB right now. Similarly, I'm about to set my dad up with a Synology which I'll be putting 3x8TB for 16TB usable. I even offered to put in 16's but he didn't need the space and didn't want to pay the extra to get it (especially since he can upgrade later if desired)

16TB is very new, but 12TB and 14TB have been around for a couple years.

While an average NAS might have 10TB, I wouldn't even call it particularly high end to have a 6 bay NAS with 50+TB of usable space.