SM, I'm not saying fixed fortifications are "easily overrun". I'm saying they were basically useless because you could bypass a fixed fortification and just ignore it. What can it do? Nothing. Eventually those in the Maginot line gave up. And when the attackers were properly prepared no fixed fortifications ever held.
But the argument is not a defensive line but fixed fortifications. And no fixed fortifications ever really worked in the long run.
Well, we may be talking at cross purposes.
To wit, let us ask the question: what is the purpose of a particular set of fixed fortifications? There can be several answers to this question.
1) To channel attackers into more desirable locations for fighting: In this case, you
want the attacker to go around the fortifications. The goal is for the attacker to go into a different direction where your defensive forces can hand them a decisive defeat. While it was not really used in this way, the Maginot Line would have been an excellent tool for this kind of warfare had key leaders bothered to do their jobs. The Germans did, in fact, spend their time bypassing the damn thing.
2) To delay the attacker: Sometimes, you need to buy time. If you need time to mobilize or time for allies to get into the fight, then fixed fortifications might be a good answer. The Czech fortifications in the Sudetenland were an excellent example of this kind of fortification. Leaving aside the political aspects, those fortifications should have bought the Czechs all kinds of time in a shooting war to both mobilize her own forces and for France to honor its obligations and either tie down Germany in the west or launch an actual attack.
3) To multiply the fighting strength defensive forces in a region: At its most basic level, impressive fortifications should amplify the power of defending forces. We see it over and over again throughout the 20th Century. This doesn't make them magically immune to defeat, but giving 1000 troops the effective fighting strength of 1300 troops has some benefits.
The fatal flaw in thinking about defensive fortifications is in assuming that their are impregnable or that they are meant to fight on their own with no support or relief. That's not the job of large networks of fortifications. Even in WWI, the trench networks could be overrun without reserve forces assisting or counter-attacks in other areas. It's even worse when other situations preclude an effective defense. The best examples are some of the islands in the Pacific. The Japanese held fortifications (concrete pillboxes and the like) on crappy terrain, but they were defeated in every single amphibious invasion during the war. Lack of supplies, reinforcements, air superiority, and naval support meant that the islands were, in most cases, fighting alone without support. In which case, their defeat was inevitable provided the attacker continued to press. Had the IJN been on equal footing with the USN, invading those islands would have been suicide, not because the fortifications themselves were magical, but because it becomes untenable to waste time attacking those fortifications when the invasion fleet can be threatened by sea and air, and when the defenders can be reinforced and resupplied.
So, in that sense, no fixed fortifications ever worked in the "long run." But to me, the flaw is in how your utilize them, not in the idea that fortifications are useless. You don't count on them to win the war by themselves; you use them to attain very specific tactical ends.