The days after the halving tomorrow and before the difficulty adjustment in about 5 days will be brutal for fees with the Runes added in. It will likely take some weeks after the difficulty adjustment for fees to settle out to a new normal.
Why?
First, the difficulty has nothing to do with the amount of spam those guys throw at the chain.
If you think that there will be a massive block slowdown that won't happen in 5 days either, because 2/3 of the period has passed, so even if we are going to see something like 20% down instantly it will only last 6 days out of which 4 are doing the weekend.
Costly, a big clog, maybe, but in no way brutal.
Plus I'm not even convinced about that 20%, we just had 4% up previously, and even right now we're 2% in the green with faster blocks, as I said in the speculation topic, my bet is on at max 10% during the first adjustment.
My logic is that
1. We will see a drop in the hash rate once the halving occurs due to mining being unprofitable for inefficient miners, this will lead to longer block times until the difficulty is adjusted 4 days later. Longer block times means people paying higher fees to get into the next block.
2. At the same time, Runes come into play, also driving up the fees.
3. We are already in a high fee environment, this is just adding to it.
"Brutal" is a subjective term, I just see things getting even worse for a while due to the above.
I haven't been around long enough to understand the nuances of what will drive the difficulty up or down, but if BTC stays above $60k I agree that the difficulty drop won't be drastic.