I sent another email to Eddy Cue last Thursday, and received a call back from Alan L******, an Executive Liaison at Apple, that afternoon. Alan was considerate, but said Apple was not planning to increase the allotment for annual plan holders due to "business reasons". Switching to a monthly plan was the only way I would receive the 50Gb allotment (which, as we know, sells for the same amount as my annual 25Gb plan). He emailed me some links to the Apple website (e.g. "iCloud storage upgrades and downgrades" and "Changes to iCloud storage for current customers").
These weren't definitive on the matter, and I pointed to the Apple webpage where it says annual plan holders would be paying the same "rate" but was silent on the allotment one would get for that rate. I also explained why a customer may find paying a one-time $11.99 annual feed not the same as paying $0.99 monthly. Furthermore, I explained that standing behind somewhat conflicting documentation written before my email (while explaining how seriously Apple takes feedback) seems to suggest that my feedback was not truly being heard or considered by those who are empowered to resolve this situation. And Alan L. would be in a position to help explain our feedback to those who may be able to help address this. Alan was very helpful and understood that I needed to hear his answer before I decided whether to switch away.
So Alan L. agreed to take my feedback to Eddy Cue's team, and will get back to me on this. So far, I haven't heard anything further, but I at least have confidence that the feedback is being carried to the top (or a delegate of the top) and hopefully, someone will hear our voice.