a few things things:
1) if you have anyone that essential that your "bus factor" isn't adequate and you need more people.
2) And there are very few jobs that require synchronous work for the full work day, if you can't manage with an employee out on a run for 1-22 hours something is very very wrong with your process or you have a very niche job. Do you not expect people to eat, use the bathroom, get coffee, etc either?
3) There's a *lot* of research that shows that a) nearly no one is fully productive for 8 hours/day b) breaks help with increasing that productivity, giving important resets, and log term make people and teams vastly more productive
4) I'm on call at the moment 24/7 because of some new systems, for a very large enterprise platform that hosts quite a few rather important services globally right now. You can be reachable on a run if needed, I have a smart watch, and it's likely everyone you work with at least has a phone. If they're someone who's critical to reach for some reason make sure they take a phone (and set appropriate boundaries, ie only call/text/whatever if it's actually critical). Also if someone's that critical you should be subsidizing their comms, for ex I get a $75/mo stipend towards my phone bill because I need to be reachable and be able to work when needed and that covers my unlimited data and most of my general phone bill otherwise. I have the choice of a company cell phone instead but I'd rather they subsidize my plan and I don't have to carry 2 devices. If you don't do that it's unreasonable to expect someone to be permanently reachable.
5) "People are paid to work at the end of the day" - no, people are paid to get their work done. Hours worked for anything that isn't *actually* time dependent (like working on a factory line or being on station as EMS or something) is an absolutely terrible metric for productivity and the idea that someone is in front of their machine for 8 hours means they're working at full productivity for 8 hours is absurd. As I mentioned earlier in the thread my boss has a habit of noting the stat that people are most productive and can focus on something with full productivity for 3-5 hours/day. If I estimate a task is going to take 4 hours that's a full days work (the other hours in the day include lunch, breaks, meetings, documentation work around the main task, and time for out of band asks). If I allocate 8 hours of actual tasks I'll never actually get through them without working 12 hours to account for everything else - and I'll be less productive overall. I already work a lot of long days when we're in crunch time, no need to make it worse.
6) "That also creates the headache for management to make sure that employee is in fact making up their hours" - no, again, unless it's time sensitive the appropriate metric is "are they getting their work done" not "are they literally in front of their screen for 8 hours"
7) "People are paid to work at the end of the day and disappearing and loving it up at home is not part of the deal when working from home in my view" ok, but you know what? I don't live for my job. Don't get me wrong, I love my job, but my job is what pays for my life, not the other way around. My pet had a medical issue recently that I only spotted in time because I was home with them. I let my team know and bolted to the vet. Saved their life. If you don't think that's more important than work I don't know what to tell you. My team gets it and that retains staff, long run net benefit for the company too. If I had a job that told me my pet was less important than work I'd instantly quit.
8) Lastly I think you don't understand a key important metric in hiring talented engineers these days: "work/life balance". A lot of companies pay lip service to that idea, but until WFH suddenly became common it wasn't really actually doable. It seems like a lot of managers like you are having trouble coming to terms with the fact that people actually want what it says on the tin.