Here are some possibilities (1 or more possible):
1) The Price vs Diff vs Reward is too low for most miners, many drop out, diff drops to correct that
2) Price goes up (eventually) due to the reduction in the supply of generated BTC or other market reasons
3) Transaction fees will (of course) become twice the % of the block reward if they don't change
i.e. if there was an average of 0.5 BTC in txn fees per block, that's 4% of 12.5 but 8% of 6.25
This % of course doubles each halving, which is why PPS pools started paying FPPS, but they still charge more (and some of them don't have enough BTC to stop going bankrupt)
What will happen? NFI
But we'll find out in about 5 months or so.
Don't forget, this will be the 3rd halving, we've survived the previous ones, this is not new.
I personally have rewards in 2018 which go up to 0.00007 per block which is about 50 cents. But that is just me and that is just one block out of like 130 that I got dust for but never actually got it.
