You're mistaken, I'm afraid. Duel Monsters ended the same way as
in the manga. To be perfectly honest, for any series with which this
has happened that I've seen, I would have preferred to see it be put on
hiatus until the manga was finished. But the anime adapters do need
their money, which is sad. This again brings to mind Soukyuu no
Fafner. Instead of possibly squandering the movie by rushing it,
they're instead presenting us with this movie, a little unexpectedly,
years later. Some of the fans may have long dropped the series, but
those who were truly willing to wait for it, like myself, have checked
for news on a fairly regular basis.