How so?
Col Ben Anderson (Ret) and LTC James Behnke (Ret) (among others) have stated how this could be done with a lot less bodies than you imagine would be needed.
Look up "McNamara Line" and "Vietnam" and get back to me.
Combine soldiers with physical barriers (a force multiplier)
Soldiers or LEOs?
This is a nontrivial difference.
As for the "barriers," if a 37-year-old out-of-shape guy can get past the barrier, it ain't much of a barrier.
with high tech sensors, observation equipment, and UAVs (more force multipliers),
Of course, you then have to provide security guards for the high-tech sensors, because they're worth stealing.
Force multipliers are fine--to a point.
Beyond that point, they don't multiply the force nearly as much as you think. They do not make it possible for one person to be in three places at the same time.
Col Ben Anderson (Ret), LTC James Behnke (Ret), and other advocates of a low-personnel density, hig-tech interdiction approach generally do not consider that the illegal immigrants can--and will--adapt their tactics to counter high-tech countermeasures, much as the Viet Cong did with the McNamara Line.
The borders can be secured--but it's going to require a LOT of bodies to do so, and that will cost a lot of money, which will require a lot of political will. Even with sensors and UAVs, you're still going to need a very visible deterrent--and that means lots of boots on the ground.
Once someone has decided to cross the border illegally, we're playing catchup ball. A sizable force is needed to deter the potential crosser from making that decision.