It has many years left in it. While some folks like to keep up with changes, others keep the same machine for many years, as suggested by this poll, albeit with a small sample at the is stage.
https://forums.macrumors.com/thread...ou-had-and-used-your-oldest-mac-mini.1909082/
Yes, the HDD will fail eventually, and support for new versions of OS X will be dropped at some stage. It comes down to how up to date you like to be with tech.
My first Mac Mini was the 2005 G4 original. When the HDD and power supply failed after 4 years in 2009, replacement seemed more cost effective (to me) than repair. The move to Intel had been made, and there were significant advantages in moving to the the latest OS, Leopard at the time, with Snow Leopard (with no G4 support) coming a couple of months later.
The base model early 2009 Mac Mini I have now has been up graded with an extra 4 GB of RAM and Mountain Lion. I am still on the original 120 GB HDD, supplemented by an external HDD. Updating to more recent versions of OS X, although free, would bring no significant benefit to me at present. Maybe I'll another couple or three years out of it, with HDD failure likely to precipitate the repair or replace decision.
So too could a change in what I want to do with a computer.