Um, Nick, I’m pretty sure that Hitler had ordered his troops to stop shortly before Dunkirk. The exact reasons are unknown, but some suppose it was to show good will to the British.
The order was revoked later, and they attacked, but the delay still significantly helped the British.
And you talk about a few hundred thousand troops as if they were nothing. ‘A few hundred thousand troops’ are not only a significant amount for any armed force, but also a big morale factor. If the expeditionary force had experienced a crushing defeat with horrendous losses, don’t you think that might have influenced the politics?
Also, IIRC the guys at Dunkirk were, for the most part, experienced professional soldiers. That’s not the kind of soldiers you want to lose in the ‘hundred thousands’ at the start of a war, if you ask me.