We want to apply consistent policies and technical controls across multiple dev groups that have multiple AWS accounts. K8s seems to be a good solution for that.
Was it not possible in ECS? Or did your teams just want to use K8s, and decided they would use the opportunity of wanting to organize things better as a reason to switch? It seems to me that with SSO and IAM you could create virtually any kind of access controls around ECS. K8s doesn't solve the multi-account problem, and federation in K8s can be quite difficult.
I'm not a fan of AWS but to be fair it has all those features OOTB.
You can have AWS "organizations" managing multiple accounts. IAM will allow to set up any policies you want in your organization.
On the other hands side now you have a multitude of complexity added to your setup…
If I would be snarky I would see a case of "resume driven development". But let's just assume it was lack of time to find the simplest solution and it's than often preferred to just go with the flock as so many others can't be possibly wrong.