Agreed. BIP100 will give miners the power to increase or decrease the block size every three months, much like how central banks can fiddle with the interest rate every quarter.
Also, it doesn't seem to make sense for BIP100 to give miners the ability to decrease the block size limit, unless the intention is to give miners even more influence over transaction fees.
In the real world, if demand for a commodity or service falls, pipelines get turned off, optical fiber goes dark, cargo ships are mothballed... because there are real maintenance costs in carrying excess capacity.
However with Bitcoin, if transaction demand falls, miners can simply mine smaller blocks. Given there is no penalty in producing blocks smaller than the block size limit, why allow miners to decrease the limit?