I'd agree that defining 'melody' gets you into a hornets nest, so best not to try. It's even more complex than beauty, which 'one knows when one sees'. The theme of the allegretto of Beethoven's 7th is surely not a tune. However, the movement itself is memorably tuneful. Beethoven is invoking a sense of melody even when his theme is not a tune.
It is of course the right of any composer to use or not use one or more tools of the trade. It seems to me, though, that the 'absence of melody' was in some schools of 20th century composition elevated almost to a religion, as though incorporating tuneful elements in a composition was heresy.
Obviously, a great composer will not feel bound by constraints of fashion. It is possible that some composers who chose to expunge melody from their tool box will emerge as 'great'. I doubt it, personally, but that's just one guy's opinion.