But kafka isn't a standard message queue, and therefore won't make onboarding easier except for the ones who already know it, which is not a significant majority, right?

Or at least we'll have to agree on the definition of standard here. For the equivalents of SQL, I'd say we have AMQP or MQTT, and kafka does not seem to support any (my goggling tells me it has its own protocol); deployment-wise, it doesn't seem easier to set up than, let's say, SQS; while used and supported in most common languages, most queue/job frameworks do not seem to support it OOTB (looked into ruby, python, php, javascript), so you'd have to write your own custom kafka handling code from scratch. So what would make it easier than just riding postgres?