At some point, owners of older hardware will have to admit that they aren’t getting any more software updates and will need to get a newer device. Typically, smartphones have a 2-year life cycle nowadays, so the Desire 816 should be good for a while more, but its users will have to start looking for something new sooner or later.
For what it’s worth, HTC are one of the more responsible companies when it comes to keeping their devices up to date, and they have a difficult job with that too, as their version of Android is quite modified.
It’s not as easy as repackaging the new update and releasing it for their own users, as in most cases HTC have to do a lot of modifications to Android to make it usable on their own phones and get it to look like their own Android. In the end though, their fans love their custom Android version, so they’re doing their job right.