Terminal Total Difficulty (TTD) – Understanding Proof-of-Stake and The Merge

The effect of the difficulty bomb reveals itself slowly at first and becomes exponentially more obvious over time (as you indicate). Without a hard fork, the network will grind to a halt, which (as you also indicate) forces everyone to move to the upgraded code. Its effect is just starting to become noticable: https://etherscan.io/chart/blocktime.

The pending release of Metropolis contains many fixes/upgrades (detailed in the EIP repo) and lays the groundwork for the switch to POS, but it does not yet implement POS. It is my understanding that Casper, which is also not ready, is either required or at least highly desirable to implement POS.

Of course, it's impossible to predict the future, but I would guess that part of the Metropolis release will reset the difficulty bomb to a later date that will extend far enough into the future to allow for Casper and POS to be properly prepared and tested.

Summary: Difficulty bomb is already exploding (slowly), and will be reset with the Metropolis hard fork which does not include POS or Casper.

Each block has a difficulty.

totalDifficulty is the sum of all the difficulty of the block and all previous blocks. Another way to describe totalDifficulty is "cumulative difficulty", but cumulative is a longer and not necessarily simpler word.

Even if hashrate decreases, difficulty is still larger than 0. So totalDifficulty will keep increasing and eventually exceed TTD.