If we're appealing predominantly to those who would use youth hostels and want budget travel, yes absolutely. But thats not the market for the cabins and indeed, thats who the seats are intended for.
Couchettes would put off a good range of people. My partner wouldn't go anywhere near, and I'd be incredibly hesitant (and I'm quite happy to stay in a youth hostel).
You've got to bare in mind that those who are prepared to pay the (quite high) fees for the berths are more likely to spend a good bit of money in the lounge car, all of which adds to the viability of the sleeper.
I'm not saying couchettes are a bad idea, but when the sleeper is filling most nights a good month out, what is going to be achieved here? If the pricing structure is going to be as you suggested, what happens to the seats (who would use them without a price cut)? I think if I was going to do anything, it'd be strip out the seats and add pods in their place if you can get roughly a like for like number in their place - but again, you'd have to reassess the price structure and even a like for like number would (and should) cost more.
99% of the time I have absolutely no issues. Obviously sometimes sleep comes easier than others, but ultimately you are still on a moving train - so don't go expecting the best night's sleep! Having once paid £500+ as a surprise treat for the double room, I didn't leave feeling like I got poor value for money.