I'd love to have more a mix of WFH and office long term but that link above about the problems associated with this were clear to me from about May 2020. If you want a mix of working from home and in the office, teams will have to work to the same schedule eg Mon- Weds all in , Thur- Fri all at home or whatever split is decided on. Another team can then use the office space on Thur-Fri. Anything else leads to hot desking which many people loathe, promotion for those less often in the office and chaotic meetings mixing zoom and in person which never works.
Even fixed office days within teams is a nightmare puzzle to work out taking into account team sizes, space available, team work patterns, space for new employees for team expansions, reorganization etc.
A free for all where employees can choose which days they work from home is unworkable and even having some set days for WFH is complicated.
Why would employers make their life much more difficult logistically than it needs to be? I think most companies will go back of all in the office , with a few making the change to all WFH as those options are both easier to manage .
Some employers might allowed a limited flexibility to work form home for some employees in some instances. This happened pre Covid too but maybe a few more employers may be open to this in the future.
So overall, after Covid is under control , I think we will all be mostly back in the office with maybe a few more companies having most staff WFH and maybe a few more offering a tiny bit of managed controlled flexibilitiy to employees. Some may even try the fixed days in the office and WFH for different teams
What is definitely not going to happen is that we will have a utopia of all office workers mixing home and office working in a ratio to suit the employees, much as my little socialist, anti-capitalist heart would love that 
