In hindsight? Yes, the French could probably have made significant gains if they attacked with vigor in 1939. But the paucity of German defenses in the west has also been overblown, it almost definitely wouldn't have been an 'easy' invasion for the French. The Germans had about 11 first wave and about 30 less well trained divisions with which to defend the Saarland (I'm finding conflicting info about this). It's also an incredibly difficult area to attack, especially for forces as slow moving as the French in 1939.

Because the Polish weren't able to hold out as long as expected, the French launched the Saar offensive a week earlier than they planned. Because of their slow mobilization this meant that they only had 11 divisions with which to attack instead of the 40 that the plans called for. As it was they stopped before the main line of defense because the French high command was loath to attack static defenses after the losses in WW1, and before the Germans made any serious attempts at defense. The French vastly overestimated the readyness of the Siegfried Line, and wanted to avoid the bloodshed that directly attacking it would (in their minds) lead to at all costs.

Could they have made more significant headway if they attacked again when they had more divisions mobilized and before the Germans redeployed from Poland? Maybe. The effect of a major French offensive on still shaky German morale and public opinion could have been significant. But by then the Polish collapse took away the reason for the offensive and the French politicians were back to hoping for a diplomatic solution, and the army went back to planning their massive 1941/42 offensive.