You are being reductionist, and only accounting for the factor of direct eletrical consumption during use, and not the total energy and natural resources spent during the equipments lifetime , specially its manufacturing (which i think is the real measure of ecological efficiency we should aim).
The old hardware was already produced, resources were spent and pollution and emissions were made, so it is a duty that we should use it as much as possible until irrepairable breakdown happens, and reduce our consumption of new stuff that would only replace it. Like your example of a new smartphone or new steamdeck replacing the old Xbox Original: it is almost certain that the electrical consumption gains would not compensate, in terms of total energy and resources spent and pollution made, the discarting of the original xbox (in fact, it would generate more waste that would need to be reprocessed, generating even more pollution).
Remember the 3Rs ? The 1st R is REDUCE, not ''replace stuff every 3-5 years to get slightly less eletric consumption''. It is kind of the point of the Permacomputing movement the OP mentioned: perma comes from perma-nent, make computers that last much longer lifespans than the current system. If people could repurpose old PS3s and Xbox for playing indies, and bigger games we demaked to it, people could keep using the stuff and buy less.
A Few texts for inspiration: https://solar.lowtechmagazine.com/2020/12/how-and-why-i-stop... https://www.iltascabile.com/scienze/sostenibilita-digitale/