We check for domain expiry and then trigger an alarm in the monitoring system when the valid.date.days <= 90

I do this for SSL (<= 30), but I should probably do it with the domain as well.

Can't hurt. =)

I'm confused, how is it over-engineered?

This is the job of the registrar

Turns out, sometimes someone doesn't do their job, or someone ends up in the hospital, and sometimes weird things just happen.

There are lots of places where we may want redundancy to redund at us.

I wasn't trying to justify otherwise?

Over-engineering is when you make something to be more robust than is necessary for it to work successfully. Evidently depending on the registrar actually failed here (just as it has for many in the past), and this will surely happen again in the future. I don't see how making your system robust to failures that actually come up is over-engineering. It sounds more like just plain old engineering. (And I honestly also fail to see what is productive about pressing on with this conversation.)

You don’t pay them to do it. It’s not their job. In fact by default it is nobodies job, it’s just your own responsibility.

The registrar may see it as an opportunity to sell you another year of service but it is not their job.

It's their job. But it's your ass if they don't do it.