The trouble is that there is always going to be a perimeter between the "secure" and non-secure areas no matter how far you push it back.
That's true, but at a perimeter fence, there could be several independent checkpoints, spaced a few hundred meters apart. An incident/explosion at one wouldn't affect the other checkpoints, so wouldn't stop the airport from functioning.
As the perimeter expands, there's a diminishing returns situation in terms of security. The larger the perimeter, the less secure it is by its very nature. To secure a larger perimeter takes significantly more resources - physical and personnel - and leaves it more open to a breach.
An incident at one checkpoint, regardless of size or spacing, would necessitate the closing of the airport to make sure it wasn't a feint or decoy to allow the real attackers in. I've had the misfortune to have been in an airport twice where there was a "security breach" at screening. Even though they were 20+ gates away, the whole terminal was evacuated and rescreened. Not very efficient.
The prospect of nail bomb going off in a crowded airport lobby is scary. It would make me feel a little uncomfortable sitting in an airport for any length of time.
Any place that people gather for any length of time is vulnerable.
The idea that that sort of trouble could be kept outside, and at worst going off by a fence out in the open well away from buildings feels much less scary. Compare the numbers involved too: Airport building ~ 1000 people. Perimeter gate ~ a few people at a time, and once inside you're safe. It would be a smaller simpler transient risk, rather than the big complex risk that airports are at the moment.
It feels much less scary, but the risk is still there, and parts of it may be increased due to the difficulty of securing a larger perimeter.
You're also going to have natural places where people congregate - being picked up, retrieving luggage, waiting for transport, and so on. This attack happened in the arrival hall where people were waiting to recover their luggage, clear customs, pay their bribes, and meet their rides or family.
There were probably a number of flights' worth of passengers there at that time. Unless you spread out arrivals so that the first load is clear before the second load enters the hall, there's always going to be a crowd.