After six years of storing files on my DS412+, it wasn't even half full, and Synology hasn't yet pulled the trick I now expect from all hardware manufacturers - forced obsolescence through software updates. The old Intel processor is from 2012, but it still runs the latest DSM. We'll see how long that lasts.
So why did I upgrade? In 2018, Microsoft finally pulled the plug on Window Media Center, so I had to find a new alternative for my old HTPC. Luckily, there is a Plex Media Server package for Synology that works really well, and they've made some significant improvements to the client side in the last year or so that have made it worth the cost of the Plex Pass. The only downside was that the old NAS couldn't do any hardware transcoding of HD content which led to buffering on certain viewing platforms. The new one doesn't have that problem.