These big companies spend tens of millions on homegrown tooling, even their own languages and databases, but they can't assign one dev to write a domain-monitoring tool?

You are thinking like a developer. In reality that means that now they are responsible for it, if MarkMonitor messes something up they can use their relationship to all the registrars to fix the problem and MarkMonitor is on the hook in case anything goes wrong.

This is a better situation to be in than some internal tooling that failed to notify someone because it got forgotten after the developer left.

Because it's cheaper and more reliable to outsource that to a company specializing in it.

If one dev had written it, how many times would that tool have failed by now? When the original dev left the company a decade ago, the tool has been transferred between teams six times, it failed a migration and the email address it used to send errors to no longer exists so nobody noticed, and it's literally gotten lost in the shuffle?

Markmonitor is much more about the people and service behind it rather than the software. To replace markmonitor you don't need a dev to write a tool. You need a dev to write a tool, and then a team of people who build relationships with everyone in the domain world and are available 24/7 to make calls and deal with issues if they come up.

It’s one of those ‘this problem is so simple, our big corporation cannot hope to solve it’ type of problems.

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