Remember that the French alone are definitionally going to be able to do less than the UK and France (plus others) were able to do together. If UK and France together couldn't cut off the Baltic supply lanes, it's extremely doubtful France alone could have, even if they were assured of no Italian intervention (the basic premise of the 1v1 that no other power attacks the two combatants, not exactly realistic, but that's the scenario that was being argued).
Sending submarines into the Baltic would be very risky. The Danish straits are too much of a chokepoint for mines, submarine nets, and destroyers. Not to mention the Germans would have air cover over the Baltic.
The British considered sending a surface navy to the Baltic, but the plan was abandoned for being considered too risky. Also, ships in the Baltic can just hug the Swedish-Danish-German coastlines if necessary (violating this neutrality risking diplomatic repercussions).
Soviets are going to be happy to see the French and Germans fighting a re-run of WWI. That was their whole plan (let the West bleed itself dry again and then swoop in to pick up the pieces). Obviously the defeat of France nixed this plan in our own history. Germany had shown it could cut a deal with the Soviets to buy time and resources in exchange for splitting Eastern Europe.
As for the Maginot, again, Germany could just go around it through the Low Countries. Or employ some of the innovative tactics which they used in capturing forts during the Western Offensive such as glider aircraft and paratroopers combined with heavy artillery and combat engineers.
Beyond this, WWII showed the power of an air force. Germany's air force was focused around supporting ground troops and vastly outnumbered their French counterparts.
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Regardless, I digress to the thread's topic. The issue is, as others have said, no one trusted Hitler, and that leaving Germany in such a dominant position would allow them to turn Eastern Europe (including the remains of Poland) into their sphere of influence, all the while giving them more opportunity to build up for further attacks. Besides, for the Germans to be doing well enough to have brought the Allies to the table, they are likely going to be demanding more than the Allies will be willing to stomach.