Where is the digital ID required? Because they can’t suddenly require that all computing devices in existence support it.

Is it only for accessing major social media websites? Only for device sales?

Because it seems to me neither of those will make a significant dent in use of older devices; might even drive more people to the Fediverse.

Yeah. This is only required for businesses in most of the EU. For private usage they can't really force it. So your job may require you to install an app if you have a work phone or computer. Your private useage is, for now, still private. Also, in Germany at least, they just passed new legislature so that people can get replacement parts (batteries, screens, etc.) For their phones and PCs much easier to reduce he amount of electronics being thrown away because they become outdated. That also includes the availability of custom Roms such as Lineage, E/os and Graphene for android phones. The use of Linux has also never been an Issue in most EU countries. Most businesses actually use forks of Fedora instead of Windows. The French government recently switched from Windows to Fedora (I believe). So while your claims are mostly valid, for businesses, any plan of this magnitude for personal and not business use would take years and have to be approved by each country and it's citizens.