The problem with isolating high risk populations is that you have to go round them all up and temporarily house them in quarantined facilities. And since the elderly are sometimes infamous for their unwillingness to be rounded up and forced to leave their homes, you have to either force them anyway or just abandon them to their fate. And that’s without touching the massive logistics of such an effort. (Not rounding them up would be even harder.)

Honestly, if you were gonna try and do that, I think geographical isolation would be a better option. Compartmentalize your state/region/country into separate zones, block all non-essential travel between zones, regularly test essential travelers, and change the lockdown status of each zone based on local conditions.

What will end up happening is occasional breaches between zones where a zone might go from green to red. But it gets us in a position where most people are mostly unrestricted most of the time. It also makes it possible to eradicate the virus without actually infecting most of the population, which is nice. Logistically you’d, at most, just set up checkpoints on roadways and inside airports and train stations to enforce the travel restrictions.

Over time you could even allow travel between green zones.