The difficulty bomb exists to make evolving the protocol by forking it smoother, since there's no practical way to keep using the 'original chain' indefinitely (TBH I think this is one of the better ideas in etherium after seeing all the drama around bitcoin hardforks). It can be effectively put off indefinitely while still staying on PoW by just forking to a version of the protocol which the same but with a later difficulty bomb, and indeed this has already been done multiple times. The current plan as stated by the foundation is to not extend it again (at least officially: miners could start their own fork if they wanted but it would be up to the market to decide which one is worth anything), but they can postpone it again.