Given a chance would you kill baby Hitler?

[Anomalocaris]

The so called "Sichelschnitt" certainly sruvived contact. It worked even better than Manstein had expected.

Perhaps the Allies simply worked worse than expected? 

As for Dunkirk there are lots of contradictory explanations about that from Hitler's own WWI recollection that the terrain in the area was unsuitable for tank warfare to the very probable suggestion that the idea originated with Von Rundstedt and not Hitler.  German infantry divisions largely had horse-drawn artillery and did not move anywhere near as fast as the panzer divisions.  Re-grouping the panzers for a drive against France while giving the infantry time to move up their heavy guns does make some sense.   Add in Goering's bullshit about the Luftwaffe's ability to prevent an evacuation  and you end up with a situation of counting your chickens before the eggs hatch.

Hannible's plans also worked as far as we can tell perfectly at cannae.   A plan that is far outside the conventional notion of what is plausible, executed very forcefully and quickly, fast enough get inside the enemy's observe, orient, decide and act loop to prevent the enemy from adapting, and backed by high quality staff work that prevents rapid build up of internal friction, usually survive encounter enemy sufficiently intact as to remain clearly recognizable until it attains its objectives.

From 1939 to early 1943, the most important element German military successes against enemies of comparable or superior strengths can almost all be pinned down to the ability to get inside the enemy's OODA loop.   From mid 1943 onwards, it was the Soviet who were able to consistent get inside German OODA loops.  That was probably the underlying reason why war on the east went from German tactical supremacy to Soviet tactical supremacy so quickly, with very little in the way of a period of evenly matched stalemate.