The Siegfried Line was made enormously more effective by general malaise in the Western armies (then under French command) though; in spite of being relatively weak and unfinished it went largely untroubled. There were some small incursions into western Germany but they weren't substantial.
What makes this particularly galling is relative strengths of the two armies at the time; the Allied forces in northeastern France had significantly more tanks (for example) available there than the Germany army had in total, let alone defending the western front. It's far from impossible that a serious, concerted attack on western Germany in the 1939-1940 'phoney war' period could have had enough success to significantly alter the course of the war and turn it significantly into a land war in western Germany for much of the period.
Its very true that it wasn't yet as strong as it was designed to be nor was it as strong as the Maginot line. It only had to be enough, though. If Germany's western border had been completely unfortified there would have been no way that the Allies wouldn't have attacked. And even the unfinished wall represented a huge investment for Nazi Germany. If you haven't already I'd recommend reading "The Wages of Destruction" on the economy of Nazi Germany, its a fascinating book for a number of reasons.
What makes this particularly galling is relative strengths of the two armies at the time; the Allied forces in northeastern France had significantly more tanks (for example) available there than the Germany army had in total, let alone defending the western front. It's far from impossible that a serious, concerted attack on western Germany in the 1939-1940 'phoney war' period could have had enough success to significantly alter the course of the war and turn it significantly into a land war in western Germany for much of the period.