Your plea (along with the related ones of @erlangga, @zwhitchcox, and others) has been heard, it seems! The beta of per-app egress IPs was just announced:
https://community.fly.io/t/beta-feature-try-out-app-scoped-egress-ips/26536
There are still some limitations, like really being per-region rather than strictly per-app, still a distinction between ingress and egress(?), etc., but it looks like a much closer approximation of the ideal…