You're assuming Nintendo used AWS instead of colocating their own servers, but even if they did, "just firewall it" doesn't cut the mustard. The service itself is getting shittons of user data. If you manage to compromise the old servers, it doesn't matter if you can't get to newer stuff, you can still attack a bunch of people playing old systems[0].
You might not personally think your Wii U play data is worth securing, but the GDPR does.
This doesn't sound like a real issue. They could just firewall it off in a separate amazon VPC or publish a docker / vdi version of it, etc.