Peoples needs change over time... data storage needs typically only increase overtime as your family grows. Depends on your mindset though, some people are fine deleting stuff they no longer need/use, other people like to hoard data
2x 16TB drives is roughly $1300 by the looks of it today. Even with it being mirrored in a 2-bay NAS, that's a lot of data you're trusting to a single drive. Should one fail, you'd have to have $650 on hand to buy another drive.
Comparatively speaking, you can get 4x 8TB for about $1200 and run them in RAID6, offering you protection against 2 drive failures while still having the same 16TB of usable space. Should a drive die, you're looking at only $300 to replace, a bit more reasonable to have in your "rainy day" fund.
What I'm trying to get to, is that with a 2 bay unit, you basically need to start with 2 really big (expensive) drives. With a 4 bay or 8 bay unit, you can start with 2 small drives and build up as your storage needs increase. Your $200 2-bay NAS is actually a $1500 (2x 16TB) purchase out of the gate. A $800 4-bay NAS might only be $1400 out of the gate if you only needed 8TB of space today (2x 8TB) but could easily double that down the road as needed.
Home networks are going to move to at least 2.5G soon too as the new WiFi protocols are exceeding 1G. ISPs in other countries offer Internet speeds >1G as well. Better to have some foresight when buying a NAS rather than only thinking in the moment as it's typically not something you really want to replace routinely.
I still like purpose-built NAS units though, they are definitely packaged much better and generally have custom designed boards. I feel they're a bit more robust than your off-shelf desktop computer.