In the following analysis, i use hypothetical numbers to show the principles of how I view our situation.
Assume our European forces have less numbers and less total 'power' than axis ones. This means that it is in our interest to fight on a narrow, as small as possible front, where their numbers don't make a difference since there is only so much soldiers you can put on a length of a frontline. This means that if we have 100 divisions and axis has 200, fighting on a aprroximate 15 province wide front(Le Grand Baguette), around 50-ish divisions can be engaged in combat at the same time without stacking penalties working in our favour. We also have forts and rivers, which would make our soldier worth 2 of them or something similiar, therefore we would be at a stalemate.
Now, if we decide to shift 30 divisions to Africa, Axis would need to shift only 30 divisions as well, since we do not have an advantageous position, and here our soldier will be worth approximately the same as an axis one. If we do this, before this situation there were 2 axis soldiers for one of ours in Europe, but after this there will be 2.5 axis for every one of us. See where I am going ? Doing this actually goes in favour of Axis forces. Indeed if I was Germany I would even try to fake weakness to give the brits confidence that they can shift troops to Africa thereby removing them from France.
Only if our forces are comparable size to axis ones, will it become feasible to defend Africa. Either that, or if Africa defense would require a relatively low amount of troops like 10 divisions or so
I understand your viewpoint however the more forces they move to Africa better because they will bleed heavily. You have to understand that more troops in Africa means more convoys necessary to keep them supplied which means more meterial and supplies destroyed by combined French and British Navy.
And fun part is that Germans doesnt have ports in meddterranean which leaves only Italy able to reasonably supply their troops still with heavy looses on convoys. And as was mentioned in WWW, ALL equipment, materials and supplies are destroyed when convoy is sunk. German convoys wont make it through English channel and Medditerranean at all. Which leaves Italy to try challenge us in Africa and if they put big amount of units there they will starve to death.
Compared to them Allies with control of the sea and Commonwealth nations adding focus on ASW we should be ok and more troops enemy has in Africa better for us.
And I agree with your analysis of European theater, if maginot line is extended and fort level is maxed it doesnt matter if enemy has double or triple amount of troops until we have at least 2 divisions in all provinces on maginot line and several in back to rotate.