You make some good points, but keep in mind that the capacity of bombers was often overestimated, and that high-altitude bombing didn't become efficient until several years into the war, when the allies has massive air superiority. Assuming the trench-war in mind had roughly equal industries supporting them (i.e. no United States), the defensive benefits in terms of air war, best demonstrated by the battle of britain, should limit the destructive capacity of low-altitude bombers. You have to contend with enemy fighters, that are closer to their bases and can land without their pilots being captured, AA guns on the ground, decreased knowledge of terrain and the targets you are pursuing, radar giving the enemy real-time information on your location, fighters are cheaper to replace than bombers etc etc.
You misunderstand me.
Success of the bombers is not even necessary.
The fact that they exist means that significant force have to be seconded to defend the rear areas.
This was not necessary in WW1.
If the supplies are properly organised, most trains could travel by night or at least have AA guns on the trains or around the tracks. Most of the "rear area" you speak of doesn't need to be protected by groundbased AA, only supply-depots and transportation.
Again, the fact alone that the effectiveness of the rail network is halved if one wants to be safe is drastic divergence from WW1.
Also not only AA is needed (and supply depots and trasnportation is not such a small area) but also garrison troops to defend against para troops.
Furthermore, transporting supplies and troops towards the defense is at least equally difficult compared to transporting towards the attack, assuming neither side has total air superiority. Well prepared defensive position should be harder to bomb than trucks stuck on a road. During their advance into France, the Germans were often extremely vulnerable since they were trying to advance rapidly, but the french and british were unable to exploit it. Since an advancing force lacks the defensive bonuses of their enemy in terms of air war since they have had no time to prepare defenses. This never came to bear during the fall of france due to lack of low-altitude bombers and fighter escorts on the allied side, but assuming parity in air power, offensives should be more vulnerable to bombing than defenses.
The problem is another. Defence is static.
By the end of WW1 every square metre on the western front was registered or close enough to registered point to be easily hit.
Any static defence faces the same problem:
The enemy knows where you are, while the attack does not need at a certain point.
If we assume air parity (which seems more likely than the air superiority you assume) both sides are equally blind, but not equally liable to be bombed because the attacker can hidden, displace or withdraw his troops if they are spotted while the defender can't.
The defender needs to stay where he is.
So when the attacker starts his operation he can smother known positions by artillery and aircraft (artillery does not kill as much as blind and suppress, same as aircrafts) and push his troops in fast enough to eliminate defensive air (if attacker and defender are within 500 metres of each other aircrafts of either side will become quite scrupulous about attacking) and defensive artillery.
The problem with trench warfare in WW2 is that what made it viable in WW1 (impossibility of breakout after breakthrough, problems with coordinating artillery, invulnerability of the rear area, the allied always pushing) are no longer available in WW2.
In that case those (roads etc) are strategically important locations and thus part of Festung strategy. I did say that all strategically important locations would be part of the Festung plan. You notice that i said "like" and "other important places", meaning i did not list every single important strategic objective.
In that case you have lead the Festungs plan ad absurdum.
Now EVERYTHING is part of the Festung, or close enough that it does not make a difference, which destroys the whole advantage of the Festungs plan, namely concentration of force.