I'm not saying it's perfect. I'm saying it gets around the corona virus issue.
I also don't think it will be as costly as people think especially if you structure your work properly.
I've never had downtime at a job. That indicates that there is always work to be done at most places. If you have a few too many workers then you utilize them effectively.. and you might as you said, send someone home early.
If you end up with too few I've seen that most businesses survive a busy day. You schedule a few extra over the next couple days (invite more workers to come in) over the next couple days to get caught up.
But if you do this it would be helpful to have the business open more hours a day because everyone's desire to have exactly 40 hours and it's exact correspondence with the number of hours the business is open a week doesn't allow you to over hire available staff while giving everyone 40 hours.
I think businesses should open more hours anyway with two crews that overlap. Why? Because if every business is open the exact same hours and it's all the hours that people are working.. then workers can't access businesses. If you've ever worked a job where you work 10 or 12 hours a day 6 days a week and you realize you can never go to the bank (back when that was important) you realize the stupidity of everybody having the same operating hours.
I'm also against 24 hour operation because night shifts are unhealthy for workers. If businesses could break tradition and see the value in an 11 hour operating window where some workers come in an hour and a half earlier than tradition, and some workers come an hour and a half later than tradition, and all employees have a five hour overlap (giving them more than enough time to coordinate with each other). Then all people will be able to access all businesses either before or after work. This also guarantees overlap with other employees in other organizations that you may be expected to work with.
The traditional hours model does serve a purpose. A business must be certain that a certain minimum number employees show up to get the work done, and at the same time it to make sure that there won't be so many workers showing up that it is wasting payroll.
My workplace has flexible quitting times in which whomever has been on-site longest is welcome to leave early if we're slow and the important work is done.