Talking about my company, the one I've direct experience about, the problem is not so much about the existing employees, but about the new hires.
Being a technological company, and having already had (due to quality certifications requirements for our type of activity) a full plan to remain fully operative even in complete absence of the office, at the start of the pandemic we simply had to implement it. Also, some of our employees living more distant already were used to work at least part of the week from home.
In the past two years the only employees that make regular use of the offices are the technicians that need to use the laboratory and the storage of components for our customers, and we had absolutely no problem keeping our work efficient, in many way even improving it (as an example, by the use of teleconferencing we can start an impromptu meeting without having to reserve a room, wait for everybody to reach it, lose time coming and going etc. etc.).
But we could do that because we already know each other well, everybody know their job as well, and we are generally at the moment a quite selected and reliable team.
Introducing new employees in the existing teams is instead quite harder: sitting side-by-side on a computer and doing a spot of pair programming is quite different than doing a remote collaboration session, for a new customer service operator having some of the more experienced personnel listening with at least one ear to all their calls for the first months is very helpful, and for all technicians the coffee breaks were usually invaluable opportunities to talk shop, brainstorm about the latest technologies and share stories, and generally getting to know each other.
We have a couple of new employees that we had to hire even in this period, but we are really struggling bringing them up to speed.
And it's hard for at least some of the employees too... most of us have perfectly fine home setup for work, but especially some of the younger ones would much prefer our office comfortable ergonomic chairs, multiple large monitors, high-speed fiber, ac, swimming pool, park, table tennis, etc. etc. to having to work from their bedroom.
I don't think we are ever going completely back to pre-pandemic work setup, but I also think that as soon as it is possible, we will go back to at least an hybrid system.
And by the way, we have already seen that even simply for diplomatic/commercial purpose, we have to start to go in person to meet customers: simply, "smothering fires" or closing contracts does not work nearly as well in teleconference.