> What about when the one dev deploying their jar to an ec2 instance moves on to another company, how does the next dev even understand what this jar stuff is

I think that's more a problem of one person vs a team, as opposed to deploying JARs on EC2s vs cloud and a bunch of custom tooling.

If you have a team of devs and they all deploy JARs to EC2, one of them leaving won't be a problem, the rest will still know how to do it. If you were to have a single platform engineer who's built a bunch of custom tooling over a bunch of Kubernetes files, and nobody knows how it works or where the files are, and then they leave, you've got the same problem as the solo EC2 dev leaving.