There have been some studies on this, turns out employees will give up quite a bit:

https://www.library.hbs.edu/working-knowledge/charting-remot...

https://anderson-review.ucla.edu/tech-workers-take-much-lowe...

Just left a comment elsewhere (https://news.ycombinator.com/item?id=45192176), but it's likely this RTO push is partially to renegotiate to account for this perk.