To correct and expand upon your point a bit. Yes, the Super Hornet isn't just a limited upgrade.
The Super Hornet is a bigger airframe. You can think of it as "what would happen if we scaled up a Hornet by about 15%?" New wing. New intake design. It required a development program and a flight test program of a kind that you do when you make a new airplane, not upgrade an old one.
The Super Hornet is much more capable. Greater endurance, can have an AESA radar, better ESM suite, better communications.
I guess it depends on how you define limited. But regardless, the minor low-observability upgrades in the Superhornet were insufficient to make it survivable.
They replace (iirc) 30% of the plane and it's much bigger.
https://qph.fs.quoracdn.net/main-qimg-2f54130dd50490fcfd1ad0...