Quite the opposite. The trenches weren't simply a bunch of holes dug into the ground, they were a system, a machine, designed to marry logistics, operations, and defense in depth.

The trenches were designed to be defensible. If an enemy took a section of front line trench it was still an uphill fight to take more of the trench. The trenches were built with machine gun nests providing coverage so that enemies in the trenches could be mowed down behind pinch points without being able to advance deeper into the trench. What's worse than running across an open field with machine guns and artillery trained on you, all of which has been dialed in with fields of fire and precision targeting? It's machine guns and artillery trained on you down a tight trench. Further lines of support trenches and reserve forces meant that invading forces often had to contend with losing local numerical superiority very rapidly.

Meanwhile, the defending forces exist on the tip of a logistical supply chain which funnels ammunition directly into their laps while the attackers are cut off from their supplies for a time. All the while the clock is ticking as railcars are moving in the background to shuttle hordes of reserve units directly onto the doorstep of the breakout.

This is why the stalemate lasted so long during WWI, it was enormously difficult to exploit and expand a small takeover of an enemy trench. The attackers would be cutoff from resupply and at a disadvantage while the defenders would make use of their existing defensive facilities to prevent the breakout from spreading while reserve units were being moved to make a huge counter attack.

It was really only the advent of modern combined arms tactics that made it possible to overcome the trench system late in WWI, but even that was substantially aided by the severe shortages of especially German materiel late in the war due to interdiction and blockades and so forth.

In the early stages of the War there was a realization that barbed wire, machine guns and bolt-action rifles created a huge obstacle to any assault. Over time, as the trench system was more developed, some of that defensive advantage was offset by massive artillery barrages that would degrade the defensive line enough to allow an assaulting force to be able to get across No Mans Land in enough numbers to be able to take the forward defensive trench. Yes, there would be other factors- how meticulously the attack was planned and timed, complications like poison gas and tanks...but massive use of artillery would be the focus. In the start of the Somme offensive, for example, the British fired 1.5 million shells over just four days at a 25,000-yard stretch of the German line.

However, those assaulting forces would be degraded- even the massive Somme barrage failed to cut up the barbed wire in front of the British forces. And it also did not succeed in doing more than badly degrading the German forces, because the Germans had arrayed their forces in depth: they had plenty of men kept back from the very front line. The French and British assaults that were successful in carrying the German front trench soon found themselves under attack by those German forces moved up from the rear.

The defenders would have the advantage of what's called interior lines : they could easily move or shift supplies and troops into the front, the attacking troops found it was very hard to bring forward the massive amount of supplies and men in time to reinforce them. This transport problem was new, and difficult to solve; fast road-building across a shell-battered landscape was not easy, nor was it easy to coordinate the flow of supplies from rail-head to supply depot to motor- and horse- transport. As a result, all the armies would be greatly damaged at the end of the Somme offensive, and the British and French territorial gains were nil. Germans would have the same experience at Verdun.