I still use a Late 2009 MacBook upgraded with 8GB RAM and a 256GB SSD as my personal laptop. Is fast enough for my personal use, which basically is web browsing, streaming from Plex, light photo sorting and editing, email and office documents. Thanks to the RAM and the SSD it feels snappy (despite the slow C2D CPU) and it still gets software updates.
I think I'll keep using it until it dies or no longer gets OS security updates, whichever comes first.
I've been tempted to get a newer mbp many times, but mine does work, it's trouble-free and fast enough for my use, so I haven't got around to spend 2000€ for a new one. It also has great sentimental value since I've had it so many years and I've done so many things with it.
At the end, YMMV, depending on the software you use you will need to replace your mac more often, but for an average user, computers have been fast enough for a few years and if your computer is still supported and getting security updates you don't need to get a new one.
I'm running 10.13 High Sierra on my MacBookPro5,5 mid-2009. Works fine. There is a patch that gets it to work on High Sierra, with no significant issues. That prolongs the life of this machine by 2 years, since officially only 10.11 El Capitan is supported.
To make it viable, you really want 8 GB RAM and SSD, but you already have both.
The only thing is that Office 2011 has some very minor bugs when running on High Sierra, as it is not supported. It's definitely usable, but the bugs will never get fixed. Office 2016 runs on it, but it's slower, especially at initial launch. Office 2016 is also slow on my 2017 Macs, but because those machines are faster, the slowness is a bit less annoying.
10.13 High Sierra also runs fine on MacBook5,1 late 2008. Not surprising, as the hardware is almost identical, just without Firewire and backlit keyboard.
I expect 10.13 is the last OS that will be installable on these, and I expect that third party applications will support 10.13 for about 3-4 more years after 10.14 comes out, so hopefully these things can last until 2021-2022. If so, that means 13-14 years for MacBook5,1, and 12-13 years for MacBookPro5,5.