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 and are 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?
How do you do it with miners? How to determine which one to include in a block? Both transactions are valid.
How is this double spend proof?
BTW: The chain can still split in this case.
Ask the Ethereum devs how they'll solve it with their PoS.