I don't really see an issue with adding synchronisation, there's no fundamental reason why having endpoint consumers acknowledge updates before terminating removed pods would be horrifically expensive. Especially with endpoint slices.
Additionally, there's no acks possible for clients of headless services, so just kube-proxy handling this doesn't go far enough.
But yeah, maybe accept that as a tradeoff for clusterip services, but more deeply integrate the real load balancer options.