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?
By utilizing the Longest Chain Rule.
So you admit that
Instant X transactions shouldn't be trusted without at least one confirmation by the miners?
That's not at all what I'm saying. What I'm saying is your proposed design "only the masternodes write the IX blockchain" can't work because there would be no "longest chain" anymore.
Do you see the problem here?
I see you creating strawmen, unwittingly or intentionally. If I had to guess it would be the former.