Getting a message must be mostly a surprise. One daily message is OK for me, but if so it may be good to tell about more than a backup. A daily digest of "everything is up and running" may indeed be something very nice to have.
And how do you ensure a team will always have someone surprised not to get a daily message? Maybe sending for may people, or requiring some acknowledge, and creating a warning over the warning channel if there isn't one. (And you certainly aren't going to send those positive messages over the warning channel, because it requires a completely different kind of action.)
> And how do you ensure a team will always have someone surprised not to get a daily message?
For a team you have a management problem not a technical one! Somebody needs to be responsible for it in the end, plain and simple.
On a less authoritarian note a technical helper might perhaps be to have a screen, always on, showing a dashboard of statuses, where everyone can see it? Make it beep incessantly when a new warning arrives until someone acknowledges it. If you acknowledge a message either deal with it (if you can) or notify whoever needs to be told so they can take action.
People will still sometimes ignore or miss the warnings. Nothing and no one is perfect.
But I'm only dealing with my own collection of personal data and projects here so it isn't that scale of problem - at work I used to be responsible for infrastructure as well as my actual job but we no longer operate at a scale where that is at all practical so I have to trust someone else is dealing with it (though I'm not that trusting I do sometimes take time out to check things myself where I have access).
I get a daily message, so if I know if things aren't getting sent or getting through. I've been wary of setting up anything that only sends problem messages because regularly getting the "all OK" messages proves that at least that part of the infrastructure is working.