Well, both sides get a little bit better with every iteration of the loop.
The govs smarten up a bit, catch a few more terrorists/suspects, or defuse a few more potential plots; then the terrorists learn from their mistakes, and improve their techniques and methods a bit; then the govs take that into account, and come up with measures to counter the news techniques; then the ...

Round and round it goes.
Same as with the "copyright protections" and cracks: we made this unbreakable system; it gets cracked; ok, we improve on that and create the next "unbreakable" system; guess what, after a while, that one gets cracked too; ok, this one is really really unbreakable; guess what, it's not . And so on...

So everybody is bound to get a little better with each iteration - each side pushing the other to become a little better, if they want to keep going. Nothing really spectacular, or unpredictable, if you ask me.