Decreasing the block limit (note, not required block size) in the future would not be a hard fork, it would be a soft fork.
It won't be a soft fork, it will be an impossibility. The miners of the future will be few in number and hostile to the ideas of bitcoin. This is the reality that we need to design for. Entities in control of a node will want to keep the price of maintaining a node as high as possible, so that they can control access to the information in the blockchain.