Just remember the share of mining rewards has been taken out of the calculation in HF17, so all the other shares increase by the percentage that mining rewards currently take.
I agree though, it would make sense to limit the amount of changes made in a fork so the changes produce useful feedback.
The rewards that are currently allocated to mining are being used to create a "top 20" witness position, instead of 19.
Sure, but that is quite a small change. Miners currently only get 1/25 of block producer rewards which is only 10% of the total, so 1/250 of all rewards. Other rewards will increase by less than 1/2%
EDIT: I wasn't aware of the 20th witness position explained by @timcliff. In that case, other rewards won't change at all.