No surprise when a market reaches the saturation point it slow down a bit
I'm not surprises sales are slowing down. Not everybody needs (or perceive the need of) this kind of external storage - for many, a simple external USB disk is enough, and these are also the ones for which free cloud space (and upload time) is enough.
The actual users of small NAS are heavy content producers (i.e. photo, video, etc.), people with heavier central storage needs (i..e. media players, some small business), needing faster upload and access than cloud can deliver now (and some more privacy as well) for everyday access - maybe also using a cloud service for off-site (and encrypted) backups.
Yet NAS devices can have a decent lifetime, you may need to upgrade disks for reliablitly and capacity, but not the enclosure itself, after all their main tasks are quite simple, and doesn't need early replacement. There weren't big improvements in long-term SOHO storage technology in the past years, as long as most of these devices employs some form of plain RAID, a standard file system and share protocol, there is very lilttle need of upgrading these units, since not spinning hardware is pretty reliable today for most uses.
Thereby once their target market becomes saturated it's no surprise sales will slow down. Maybe IDC wishes to push some customer cloud agenda, but the market for these devices will never be larger, but it's not going to collapse either.