If it stays here, I'm 100% on board; I wouldn't even mind defaulting to on. Forced automatic updates (hi, Microsoft) are a terrible move. I would also say that separating bug/security fixes from features and breaking changes helps, if my OS vendor had a way to auto-patch only security issues, and could ensure zero breakage as a result - no new features, no changing UX, nothing but invisible security patches - I'd enable it in a heartbeat. And they won't, so I won't:)
For a development machine, I'd honestly... be pretty fine with it. That's similar to how Arch Linux operates (but change hourly to daily or weekly) and it causes very few problems. I think I've had maybe two or three big issues caused by updates in the past year, plus a couple application-specific issues (not kernel-level or os-level). With a more extensive regression testing team I can see Fuchsia following through with the promise of seamless updates.
Windows updates also have possibly the worst reputation you can get (except, maybe, iOS update's reputation when Apple was slowing down older devices), so almost anything Google does will be better. Not needing to reboot (presumably, if they're checking every hour) after an update will also help.
If they implemented a proper A/B booting with preserving the intent/state of open apps most people wouldn't even realise an update happened and wouldn't complain.
Even ignoring the philosophical "it's my machine dammit, it'll update when I damn well want it to and not before" POV[0] it is a practical mess on[1] two fronts:
1. I use my windows machine for computation, ergo it needs to be running, and it costs me money and productivity when it is not. If it restarts to update, I lose hours at minimum of cpu-time and more until I notice and can get things restarted.
2. It makes what should be core user functionality useless. I would like to set up my desktop, with some programs open, a few spreadsheets mayhaps, browser to discord, reddit, HN, etc. Maybe a second desktop relegated to some other task. I can't do this, because windows will force a restart in the next 144 hours which will invariably fubar anything I've tried to set up.
[0]To which I strongly adhere.
[1]At least.
Having an auto-updating, fully sandboxed OS, without reboots, really doesn't sound that bad.
Especially if it's only kernel, stability and security updates.
It's not dissimilar to writing zero-downtime web services.
I doubt many people will have a problem with security updates that have no impact on the feature space.
But feature updates tend to break UX flows and not many people are happy about that.Your comment about corporate control also applies to feature updates.
I personally wouldn't want an OS that updates according to the developers wishes, with no control on my end. It reeks too much of corporate control, from simple things like changing UI or functionality to much bigger things like removing features because they are no longer in the interest of the company.