Instead what we got was Maul still looking to pick a fight with Obi-Wan. Seriously, what a waste.
Maul’s story always had to end with Obi-wan. Since TPM his fate has been tied to revenge on Obi-wan and he’s had tunnel vision for that. In The Clone Wars, he vows “we shall start with revenge” and first takes aim at Obi. First him, then Palpatine. But he never even manages to kill Obi-wan, so it communicates the same, that Maul’s quest for vengeance against the Emperor was futile. It also provides a very interesting scenario where we know if Maul had won, he would have taken Luke as an apprentice, just like he would have done with Anakin in TPM.
Maul dying in a Vader vs Maul duel would have been pure fanservice and not relevant to his arc in the slightest.