When I look back on hallway conversations, I can't think of many that really mattered. Actual important decisions are typically not made by IC engineers. Relevant conversations are usually inside team structures and not spontaneous.
Not that I’m itching to get back in an office (I’ve been remote most of my career), but this really depends on the environment. I’ve seen some really pivotal changes happen simply because someone happened to yak about it, or overheard such a yak and jumped in.
Usually these changes have been mostly technical in nature and under the purview of the engineers. But sometimes they’ve significantly changed product direction.
What!?!? I don't necessarily agree with OP, but this is the exact opposite of my experience (mostly observing others). One of the main downsides of WFH is the "always on" nature of it.
I usually left my laptop at work, and ignored my phone before (after all I'm not at work). Now you're always set up to work, so there's no reason you can't respond at 8PM. I don't, but many of my colleagues do.
Most of my coworkers, especially the ones that would be leaving early for work life balance reasons, work all hours, and I get randomly pinged at 5:30-8 by people that would just be done for the day. They don't need to commute, so they have less pressure about working at odd times, and want to show managers the advantages of the WFH thing, so it won't go away.
The effect is that it leads to a "longer" work day, but less concentrated than it would have been in the office. And similar productivity, IMO. I wonder if some of your co-workers you describe may do the same. In some ways, I feel like it's better work-life balance for some people as it's "ok" (in some work cultures) for WFH employees to take the time as they need it. And others are equally fine ensuring their work time is as concentrated as possible because they have other things they would rather do in evenings, etc.
You won't be as tempted to just pop in for a few minutes of work here or there to respond to some chats or emails.
I sympathise with people finding it hard to switch off (I was one until I did all of the above) but it's a simple problem to solve and it's absolutely nothing to do with WFH.