The 3,1 is still a very capable computer. The model specific upgrades cost little & the more expensive upgrades can be carried over to a 5,1. If you are using a 3,1 as shipped from Apple then modest upgrades are low cost & will deliver a considerable boost in performance that will see the system still useful for years to come. By useful I don't mean playing games but using it for the latest versions of the same Pro applications that it was bought for in 2008 e.g. FCP, Premier Pro, Lightroom etc
I never said it isn't capable. Different people have different needs. Some people still find PowerPC based Macs
capable. I'm saying that newer systems are more power efficient, cooler running, and in many ways faster than those old systems. Even the 5,1s are getting long in the tooth. They are extremely power hungry and can generate a ton of heat.
Sure, you can still run the latest versions of software but, don't forget, Apple recently put the 3,1 and 4,1s in to their "obsolete" list of systems. This means that, officially, Apple won't be supporting them in the next OS X release. Therefore, you may have to start using hacks to install the next version of OS X. If you don't, then you won't be "using the latest versions of the same Pro applications that it was bought or in 2008 e.g. FCP, Premier Pro, Lightroom etc"
Put a modern iMac next to these old systems and you can easily tell how much of a difference there is between modern vs old. I understand that they were great systems, but they are all way past their prime. I remember my PowerMac dual 1.42GHz G4 (the last PowerMac with G4s) choking when trying to play a 1080p video. That thing sounded like a jet engine because of the all the fans needed to cool the CPUs. Now, I have a Raspberry Pi that runs on 5V that can play 1080p videos absolutely perfectly. Time and technology marches on...