One of the things new writers are told in various workshops is that your first book must absolutely stand alone if you are to sell it.
I feel it’s one of those things where new writers are being ‘advised’ by slightly more experienced writers who think they know the ropes but haven’t sold anything either and are regurgitating rumours. Of course it depends on the genre, but I’ve read debut novels that absolutely do not stand alone. People I know have sold trilogies as their first sale. To me, it seems pretty silly, from a publisher’s point of view, to commit only to the first book. Either the marketing department likes the premise, or it doesn’t.
Fantasy and Science Fiction worlds are hard to build within the space of 100,000 words while telling a cracking story at the same time. Often, more word-space is needed to convey the story, so a large story is split into two or three volumes. A publisher would buy the concept of the entire series. Readers of SFF love series, so where’s the problem?