This is exactly what QR is doing with only offering the highest fare buckets (J as well as Y+B).

You think QR should offer I and D fares and increase the price for it.

QR is doing in another way by only offering the highest fare bucket.

I believe it is easier for QR to open or close fare buckets instead of juggling with the prices for each bucket. As this might have implication on other buckets.

I have seen EY once offering a less flexible business class fare for a higher price then the most flexible one.

Surely as an airline you do not want to do this.