Those machines should have been retired long ago because modern machines use much less power. (the Pentium4 was really bad) Of course there is a difference between a machine that you turn on for an hour once a week and a machine you leave on 24x7. The later use would pay for a new computer in just a few years just from savings on the electric bill (assuming same workload, if you then load the new computer with more things to do you can't see that)

The non-trivial environmental damage of building the new computer AND disposing of the old one should not be ignored. There are plenty of us who would rather pay a few extra bucks on our power bill rather than send e-waste to the landfill before it needs to be.

You are instead sending CO2 into the atmosphere. (depending on how your get your power, but for most of us burning things is how we get our power. Renewables are coming online fast though)

Ewaste will create CO2 and physical environmental pollution. Much of it is just dumped on the third world where some parts will be burnt, some melted down, and some subjected to chemical baths for metals extraction. Those chemicals will then be dumped.

Longevity (and certainly not performance over time) is not something I've ever herd associated with an eMachine. I think that machine might be a outlier.