But to complete my devil's advocate argument, "poached" offers could in fact be unsustainable in the long run. It may be worth it to Chrome to add a $100K signing bonus to a Safari engineer. But they're not going to be paying them an extra $100K above market year over year.
If there actually were a poaching war kicked off, you can imagine that the value of inflicting targeted damage on competing teams, and protecting teams from such damage, could balloon to a very large amount. Theoretically all the way up to expected profit, if there were no other mechanisms to prevent it.
This could have a chilling effect especially on startups entering competition with deep-pocketed big companies. They could just poach the best talent with offers the startups couldn't match. That's exactly like a big chain engaging in loss-leader tactics to destroy local competition.
You could maybe also draw a parallel with what's going on with patents right now. It's a huge tax on development, especially to startups.
Now that I think about it, this could very well backfire on all of us. Anti-poaching agreements among companies aren't the only way to prevent this sort of thing. If they get sued over that (which is targeted at the practice of poaching), what companies may start doing is figuring out more restrictive covenants in employment contracts. Exclusivity clauses, etc. I'm a little scared to think what they may come up with and standardize if their attention is focused on it.