The manner in which the breakthrough occurred and the evolving situation would always dictate the later stages of the operation. It was acceptable given the many unpredictable factors for the Germans to keep their option open. You do that often in planning. You plan carefully up to a phase line and prepare multiple alternatives for follow on stages.
The expected result, by that time, was after all for failing to make the hole, as the allies had found out so many times before.
Michael wasn't Bragation, but it was far better managed than any large scale allied offensive up until than.
And the allies did do a decent job of managing the defence. Much better than their successors in 1940.
But we've had this exact discussion before, so no point in replaying it.
Luddendorf had to deal with very limiting logistical and mobility factors, plan with a good margin of flank safety due to the lack of mobile enough reserves, and play the psychological card. (Allied non panicky response to the initial breakthrough made a dash to the Channel less attractive as a war winning move, for example)
The plan wasn't perfect, the conduct of operations was flawed, but not actually bellow par for real war conditions. Luddendorf is an easy guy to hate, but often criticised to harshly.