You said it was almost 5 years old, I just pointed out it's under 4 years, 4 years in January to be exact as it was released in 2014.
It performs perfectly for you which is great, in the same way my colleagues 5 year old laptop works perfectly for him because all he uses it for is browsing the web and playing solitaire.
Prices have increased quite a lot over the last 4 years, you could buy a high end phone for £500-600 back then, now it's over £700-1000+
Also you never bought it new to start with so maybe comparing to used devices would be a better idea, maybe you just had an exceptionally good deal which you're unlikely to find again?
You can pickup a few Snapdragon 625 tablets for around £200 new, used Galaxy Tab S2 for around the same price, a little less if you're lucky.