My opinion is that ingress portability isn't really that useful in practice. You want implementation lock-in at the ingress level, since the implementation matters. If I need to target platforms in a generic way (e.g. make something that works on both AWS and GKE), I can use Helm or some other templating solution to generate the right manifests with the right settings.
So "KongIngress" isn't a problem for me, and rather than struggle with today's situation on GKE, I'd much rather have a "GoogleIngress" with all the GLBC bells and whistles (well, there aren't many, but at least timeouts and a CDN toggle!) and not care about it being portable.