User blog comment:Emptylord/Class Reworks - Itemization Follow-up/@comment-5035765-20160512211319/@comment-1330314-20160516234216

Support Annie/Fiddlesticks were popular for a few months after Preseason 4... then weren't. Now they're rarely seen in bot lane, and mages who do happen to be able to support (i.e. Morgana, Lux, Brand, Zyra, etc.) are fine even when they do stack AP. You can't just bring one instance of preseason imbalance as an argument against a particular point of design.

The argument you make in favor of Omnivore points even more in favor to the above sustain model for Gunblade, which wouldn't "waste" the healing when you'd be at full health, but allow champions to recover far quicker after blowing their cooldowns. I think you're also missing the point of the above Gunblade changes: call it "hybrid sustain" all you want, the item is picked exclusively on AP-scaling champions (check Lolking if you don't believe me, including its extended list of top champions), so its hybrid "niche" doesn't really apply in practice. Making it a full AP item doesn't detract from its usefulness on any of the champions purchasing it, and would have the benefit of a) potentially being better on some of them, and b) working on a larger number of champions. The above "spell vamp" is also spell vamp in name only, as it follows an entirely different model, and the current Omnivore effect comes much closer to it (along with many of the flaws you yourself pointed out with spell vamp).