Lex is more than Superman's enemy. He is supporting character in Superman's cast, just like Jimmy Olsen and Perry White.
In some versions, yes, and prominently so in screen/TV adaptations and later comics that have been influenced by them. But it has not always been so. In the Golden and Silver Age comics, he was Superman's primary foe, like the Joker was for Batman, but he wasn't a "regular" cast member, just the most prominent member of the rogues' gallery. Luthor never appeared in the 1940s Fleischer cartoons. He wasn't in the 1948 Kirk Alyn serial, though he was in the 1950 serial. He never appeared in the George Reeves series. He was only an occasional guest villain in the 1988 Superboy
series. He was a regular in the first season of Lois and Clark
but was then dropped and only rarely reappeared. He was a prominent character in Superman: The Animated Series
, but plenty of episodes got by without him.
So no, Luthor is not always
portrayed as a core member of the ensemble on the same level as Lois or Jimmy. My whole point is that it depends on the version.
Would you make a Batman movie without Alfred or Commissioner Gordon?
Commissioner Gordon was hardly even there in the Tim Burton movies anyway. And not much better off in the Schumacher films.