If you start writing before you know how your novel ends you are wasting time.
Unless you know the ending, once you arrive you won’t have built in foreshadowing. Your characters will have unrealistic inconsistencies. Scenes will make the wrong points. Your theme won’t be emphasized.
You’ll have to go back and rewrite the whole thing to have any hope of making it right.
“Writing is re-writing!” you shout with glee.
And I say, that’s ridiculous.