Unless it's an extremely easy diagnosis and a guaranteed fix for $50-$100, I replace anything that's 3 years or older and even consider replacement on things in the 2-3 year range if out of warranty.
In addition to straight part costs, there are labor costs which usually are more than the parts themselves. Additionally new equipment usually means faster processing and therefore a small increase in productivity.
Most business-class PC's come with a standard 3 year warranty. Once the warranty is out, I think it effectively becomes trash if it breaks.
Too many techs waste a lot of company time diagnosing and replacing failed hardware when the cost to replace is similar or lower than the cost to repair. PC's are extremely cheap these days. Laptops are more expensive but so are the parts and the time required to fix replace them compared to a PC.
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