It is perfectly possible (and maybe feasable) to let the masternodes write the blockchain.
Why not? If every transaction is IX, then the masternodes agree already responsible for securing the transactions...
So they can just write them to the blockchain as well: let the quorums just write a block.
plus side: DASH gives a bigger incentive to masternodes
If there is a fork and two chains, how do you choose the longest chain in your proposed design if there is no difficulty and the blocks keep coming in both forks in predetermined interval?