This is exactly the key point. You either have a queue of people inside the airport, or outside waiting to go through security. The target has just been moved and is equally as accessible (perhaps more so) as before.
The point is that you could deploy this device without having a queue, just a door to slam in the face of a positive detection. Worst case, you need to space people out a bit.
Wherever you have a funnel, you get a queue. You have people standing outside waiting for the bus, smoking a cigarette. People gather and there is nothing that you can do about it.
Then don't have a funnel, or have enough of them. I've never queued, except for maybe the briefest of moments, to enter an airport building, even though you always have to pass through a "funnel" (aka draft-preventing double doors[1]).
Even the narrow automatic one-way sluice doors[2] increasingly popular as you leave the secure area of an airport only ever seem to cause a queue when some idiot tries to go back, and security has to reset them.
When there's a queue at security/immigration it's because the checkpoint is understaffed (or otherwise under-provisioned), not because there's a checkpoint.
> I've never queued, except for maybe the briefest of moments, to enter an airport building
Probably because most airports don't have security before entering. For those that do, there are indeed large queues outside. Mumbai airport is one such example.
