Regarding the problem to "decrease" the maximum block size eventually: I have thought a bit about it, I'm not an expert but I think also it would be desirable to decrease the maximum block size in the case blocks are far from being full, to dis-incentive spam attacks. It would however not be a show-stopper because the proposal is really so conservative that a spam attack would be very, very expensive anyway.

Regarding franky1's orphan risk because of rescanning nodes: I think there is no other way than what aklan said, to store the maxblocksize changes in the blockchain, so the nodes are aware of the changes when they rescan. There would be perhaps another possibility  - to make a conditional decision ("if CheckedBlockSize > ActualMaxBlockSize and CheckedBlockHeight < (ActualBlockHeight - 2016) then AcceptBlock") so nodes can accept larger blocks when they rescan and are more than one difficulty period under the actual block height, but I don't know if this introduces new attack vectors like nodes passing a fake ActualBlockHeight value.