What’s the strategy of the merge ?

As far as I know there will be a merge of difficulty bomb gradually making it harder to mine new blocks. This will happend before the PoS merge ? What is the strategy here ? Won't there be a point in time where no one will be able to sign blocks, or at least slow down transaction time ?

Btw, Vitalik also tweeted something about removing transaction from chain, should this raise some brows..?

Thanks !