There are many use cases for which a decade-old computer is still perfectly serviceable and even where they aren't, those computers can be repurposed for the ones that are.
Moreover, we're talking about televisions and old Macs. TVs with higher resolutions might come out, but lower resolution ones continue to be sold new (implying demand exists at some price), and then why should anybody want to replace a functioning old TV with a newer one of the same resolution?
Much older computers continue to be used because they run software that newer computers can't without emulation (which often introduces bugs) or have older physical interfaces compatible with other and often extremely expensive older hardware.
If people actually wanted to replace their hardware instead of fixing it then they'd not be complaining about the inability to fix it.
Moreover, we're talking about televisions and old Macs. TVs with higher resolutions might come out, but lower resolution ones continue to be sold new (implying demand exists at some price), and then why should anybody want to replace a functioning old TV with a newer one of the same resolution?
Much older computers continue to be used because they run software that newer computers can't without emulation (which often introduces bugs) or have older physical interfaces compatible with other and often extremely expensive older hardware.
If people actually wanted to replace their hardware instead of fixing it then they'd not be complaining about the inability to fix it.