The code change for raising the haircut percentage from 10 to 30% was made a while back in the develop branch (i.e. it is part of the hardfork changes). I don't believe there is any major risk at 30%. Also, I believe the change will increase HBD liquidity and reduce risk for HBD holders, and I believe this will have knock-on positive benefits for Hive holders as well.

With a small value like 30%, I don't think it is a good idea to allow it to be lowered by witnesses. It is more work and I think making it a runtime-changeable variable would dramatically increase the uncertainty involved in investing in HBD, to the detriment of the entire mechanism.