The difficulty bomb/ice age is _not_ to force the devs to implement PoS. The reason the ice age(s) exist is to make sure minority chains eventually die. Bitcoin does not have this problem as much, because the difficulty adjustment is much slower, which easily results in hour long (or more) block times for weeks/months for a minority chain. The difficulty adjustment on Ethereum is much faster, so minority chains that arise after a protocol upgrading hard/soft fork could hypothetically survive. To incentivize miners and users to switch to the new chain, there needs to be some other mechanism in place, which is the ice age. The ice age causes block times to keep rising exponentially on the old chain (if no action is taken), so miners will eventually lose their reward and users will not be able to make transactions anymore. They move to the new chain (which postponed the ice age), and eventually the cycle repeats itself.

This cycle has happened a few times already. Usually the fork would have already happened by now, so the impending Metropolis fork is different in that it's a bit late and the initial effects of the (ever worsening) ice age are felt.