Replacing concentrated and highly optimized data center servers with 10-1000X as many old desktop computers idling away at 50-100W or more would be a terrible tradeoff. That would explode the energy usage by orders of magnitude.
A big part of this is the very intense amount of energy producing the silicon wafer from Quartz ingots. While they weigh only a few grams of the total machine they reside in, they have a very sizable impact on total energy.
Funnily enough, for most desktop computers it would take about 15 years of non-stop usage to manage this. That is if powered purely by Lignite/Brown Coal. Anything cleaner, so almost any other energy source, and you have to run them way longer. If purely on solar panels and their manufacturing carbon output, it moves into the centuries range.
I could run my entire rack off of one to two solar panels (decommissioned ones from a power farm might I add). Even that would take a few years to pay for itself (when you factor in the costs of mounting and permitting) and my power company over 80% renewables the last time I checked anyway.