If you're a startup of 3 people for example 24 hours might be game over.

Yeah, I was considering them for my part time projects and some small PaaS-ish stuff. Not now.

Realistically to have 24/365 email coverage you'd need like, full-time founders or at least a couple of paid employees.

For what I was considering, I will be a "founder" but I'll still be working my day job. So effectively that is > 16 hours per day (work + sleep) I need to dedicate to the day job. While I will generally be able to respond within 24 hours, I can't 100% guarantee it.