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.
Miners must have already calculated in the drop in block reward. Some of them may turn off their older hardware at that point, but I don't expect a significant impact. Besides, it's less than 50% they're "losing". Earlier drops in Bitcoin price (around 80%) had a much larger effect on their earnings. Blocks will be fine.
I guess also the higher fees will mean less miners will turn off after the halving.
Still, I'll bet that the average fees for the 2 weeks post halving will be 50% higher than the average fees for the 2 weeks pre-halving.