The kicker is...they are just as capable as a newer machine, in some cases even more capable than some they are currently selling.
It's not the speed thats the problem. Age plays into it too.
Mac Pro 1,1s have been upgrade constrained for years. New GPUs haven't worked in them either.
You got six years out of it, which is about the most I'd expect to get out of any computer. If you want to upgrade, buy a new Mac Pro. It helps ensure the future of the Mac Pro too.
People shouldn't be expecting to get 10 years out of computers with the latest upgrades. It's not realistic at all.