Indeed, some cures come with more side effects than others. As @David cgc mentions, the main plot was an allegory of nuclear proliferation and perhaps the US military-industrial complex that Eisenhower warned about in the 50s.But if you see the future as shown in the series ending of Babylon 5 it's clear the plague was cured at some point. It's just the process of how it got cured.
The Technomages depended on Shadow tech that required the taking of life in a analagous way to the healing device, which is why Galen was estranged from them. The series would have included more of his backstory and also Lyta Alexander, Bester, and the Telepath war, I'm guessing. An unproduced first season script has the Excalibur crew tracking down Bester.