When I need to really get stuff done like merge critical changes now, in office is massively faster because you can just look around and find someone free to review your PR as soon as possible rather than waiting 4 hours for the remote crowd to get to it.
How do you coworkers expect to get a promotion and/or survive the next round of layoffs? Responding on Slack is the new "butts in seats" for bad managers, and good managers notice good communication.
If you're the type of guy that is always helping out your colleagues, that's behaviour that was often counter-productive. It too often wasn't noticed, and didn't show up appropriately on your 6 month review. But when you do it on Slack, it's noticed, or at least is more likely to be.
2. The executives are also landlords. Requiring WFO means an increase in the rental costs.