It's not a fallacy. It's an observation of the character of all those who say we shouldn't build a wall. They are all douche bags who have supported a "comprehensive immigration bill" - amnesty, in other words.
A wall isn't the only answer for the immigration problem. That's the point. So the fallacy remains. It is not a clear relation of If not A then B. Take for example Fort Knox. They use ground vibration sensors outside of their fences. These would work great along the border because they would not only detect people trying to cross the border, but also would detect people trying to dig tunnels. Then drones could be used to find the people and follow them until border patrol agents caught up with them. All of which would be much cheaper than building a 2,000 mile long 50 foot wall.
The wall is the best answer to illegal immigration. that's the bottom line. For one thing, it can't be made to disappear at the stroke of a pen when a new administration comes in. Any other scheme can be abolished simply by failing to enforce it.
Fort Knox has a big fence around it and plenty of soldiers guarding it. Ground sensors are useless without a wall. Your plan would disappear the minute another Obama got into office. Furthermore, a wall would still be cheaper and more effective. Your plan requires a lot of manpower. The wall reduces the amount of manpower required. Over the long term, manpower is far more expensive than infrastructure.
Of course, we all understand that you really don't want the border to be enforced. You're an open borders douche bag, so all your "solutions" are disingenuous.
See what you do there? Because someone comes up with an alternate solution to your's, they don't believe in the same results and you attack them. Seriously, how can you expect anyone on this forum to take you seriously? You act like you have the social abilities of a 5 year old.
You have been an opponent of border enforcement and an amnesty supporter since you joined this forum. One thing I note is that all such douche bags oppose building the wall. It's obvious that you oppose the wall because you oppose effective enforcement of the border. Any claims that other methods are more effective is pure bullshit. Nothing would be more effective than a wall. That's why Israel and Saudi Arabia both built one.
Yeah, Fort Knox has a fence and soldiers, but do they have a 50 foot wall!?!? Nope... and it is one of the most secure places in the world. Do you think with a 50 foot wall all of a sudden there is no need for border patrol agents? Uhhh I hate to break it to you, but they aren't going to be laying any of those guys off. And your 50 foot wall? Yeah people will continue to dig tunnels under it. Hell, unless people continue to patroll it, people can cut holes through it, or even blast it down in sections. This wall isn't going to be built out of Vibranium... it's concrete.
Fort Knox is a small place within the confines of a military base swarming with soldiers, tanks and other means of defense. It doesn't need a 50 ft wall. However, our border is mostly in the middle of nowhere. A more substantial barrier is required because it will take manpower defending the wall more time to reach any attempted incursions. The wall will require less manpower than any other proposed solution that doesn't include it. That much is undeniable.
We already know the wall works because we have working examples of such a wall. Anyone attempting to make holes in the wall will be apprehended in short order. Any holes can be repaired quite cheaply. Tunnels can easily be detected.
The bottom line is that walls have worked for thousands of years. Anyone who claims they don't work is blowing hot gas out his ass.