I think a fully common system introduces so many potential issues in terms of privacy and system integrity that the efficiency gains are not worth the cost. Instead, I think the best system would be something like a “virtual datacenter” - people get the computer they want/need like they do now, and when they’re not using it they can opt in to donating its compute to a common pool. Everyone can also draw from the common pool for tasks that are beyond the capabilities of your local machine, and the system tracks tokens in / tokens out for each user. The people who want/need cloud compute get it at a cost, the people who give into the system get compensated, the people who don’t want or need it don’t get inflicted with the externalities of it.
Achieving this is totally possible with existing technologies, the hardest thing would be getting a large enough baseline of users onto the virtual datacenter that it gains enough capability to attract new users. You would need a big institutional investor to lead the charge - like imagine if the baseline was “every computer at Google HQ” - but of course you then run into the problem of institutional investors inherently preferring a centralized solution that they can more effectively rent seek off of, which is why the closest to this system I can think of AI Horde which is just run by a community of enthusiasts and volunteers.
I remember the shitstorm when my middle school introduced ID lanyards that the kids were supposed to wear, but those didn’t have any kind of tracking