I've always felt their best albums had two guitarists. Loomis laid the groundwork for most of the songs I'm assuming, but their albums never felt anywhere near as "full" when it was just him by himself (which isn't to say DHIADW, EoR and to a lesser extent TOC were not still great albums).
They've always seemed to have been able to find a very complimentary second guitarist for those albums, but even if they are able to do that again, I just don't know that two non Loomis guitarists can still write songs that sound like Nevermore. As big of a blow as this is though, it would be even worse (for me) if it was Warrel who was leaving, or quitting the business altogether. His lyrics and vocal style are very cathartic.