Romero was an influential part of the original "Doom's" creation...
but I think saying he was the "genius" single-handedly behind the
game's success dismisses the efforts of Sandy Peterson and Tom Hall
(both of whom actually created more levels of the game than Romero
himself). Nor should Carmack's efforts be dismissed so readily either;
the technical aspects of the game greatly influence the creative (and
also remember, it was Carmack's D&D campaign with its unforgiving
demons that were the baseline for the setting's creation).
"Doom" without Romero would likely have been a different product, and
arguably not as successful... but I think the same can be said were
any of the other team-members excluded. Like so many excellent games,
it was the end result of the partnership that created it, not
something that leaped Athena-like out of the head of one single
genius.
Which is why I so dislike any hype revolving around any single
developer being singled out when a new project is announced. It
doesn't really matter how talented that one person is when your team
numbers in the dozens or hundreds.
Especially when that one person's last significant success story was
three decades in the past.