Why does Belgium expect France to come save them, if Belgium is the one who left the French Alliance. France post 1936 had no obligation to Belgium, and should have defended its own interests. Also, part of the Franco-Belgian border has the Ardennes forest and hills, making it easily defendable by Maginot forts like the Franco-German border is. Only half of the Belgian border is flat, that area probably just needs extra fortresses.
Either way, extending the Maginot line would have been the correct choice, because France does not have the manpower to take on Germany in an extended war, so it must conserve its manpower by making the Germans attack strong fortifications. This is reflected in game, France's manpower is too low, if you engage the Germans without forts, they'll either push right through you, or if you can hold them, you'll run out of manpower quickly. (Not counting special focus trees like the napoleonic focus tree that gives absurd manpower.)
Who says anything about Belgium expecting France to save them?
This is about not being left at the whims of an enemy. By building the Maginot-line France already made it far more likely that any German attack would automatically come through Belgium. Hiding behind a fortress line that does not take your own ally into consideration is the best way to lose an ally. It sends the clear signal that you don't care one bit about what happens to your ally, having only your own safety in mind.
You also seem to have little idea how extremely expensive building the Maginot-line was, how long it takes to plan and build such fortifications, or how much it takes to man it and keep it running. It's not a matter of going "hey, how about we extend the fortresses?" and off you go. It takes ages of putting things into law, funding it, creating the plans for it, and building it.
Not to mention that the more fortresses you build, the more of your military is bound to fixed location, meaning they are unsuitable for offensive actions. Politicians in France were also wary about the strength of the military and had no interest having a large standing army that could fall under the sway of conservative military leaders, which means the more fortresses there are, the less mobile will the army be, as the size of the army won't increase. This, in turn, means that you have no real reserves, while the enemy can just focus his forces into breaking through in one specific area, overwhelming your troops.
And as I mentioned earlier, there are no real natural barriers at the border between Belgium and France. The only decent one runs right through Belgium, which is why the whole French plan was build upon using that river-line in Belgium as further defense. And no, it most definately doesn't need "just some extra fortresses" to cover the border, that's just not how any of that works. Fortresses work when you have little means of bypassing them. That is a given when there is a larger river, mountains, or a thick forest, as all that forces the enemy into a narrow path. The border between Belgium and France is so open that you would have to cluster the whole thing with fortresses in an unrealistic manner. No one would have allocated the funds for that, no one would have accepted plans that would have thrown an ally to the wolves like that. Heck, the Maginot-line wasn't even fully equiped by the time war broke out. That wouldn't have looked any better if it had been extended by hundreds of kilometers.
The actual historic plan was for French troops to move into Belgium, join the Belgians at the Meuse and Albert-Canal, which were fortified with fortresses, and hold the German advance there. If necessary they could fall back to another river-line. Even if the Germans would break those lines, it would be so costly to them that by the time they would have reached the French border, they would have lost their offensive capabilities. it also would have meant not having to see Northern France getting ravaged again, like in WW1.
Your idea, on the other hand, wouldn't accomplish any of that. Instead of combining your strength and fighting together against Germany (France, Britain, Belgium, + others), you would let the Belgians be cast aside easily. The worst thing you could do is let the Germans pick up one smaller army after another, as it doesn't cost a lot to beat those, while you lose the strength you would have had if you joined forces. it also means that if that defensive line breaks, there is absolutely nothing that could work as new defensive line.
All of that is rather irrelevant when discussing Belgium's reaction to such a move though. Yes, it should be decidedly negative, because it is a sign to Belgium that it is on its own.
