Hi Mr Mac
I think you are quite right, any Indian service (e.g. to DEL or BOM) would most likely cannibalise some passengers from the MEB3. It would make it harder for EK to introduce a 4th daily, harder for Oman to add a second daily, and harder for Qatar to get to triple daily.
My view is that daily (or something approximating) flights to BOM or DEL on a reputable airline is preferable to the above said increases by the MEB4. Each of them offer a good range of departure times and connecting opportunities, and I don't see what additional rotations add relative to a direct service. On a slightly wider but related point, I would rather see CX implement the proposed additional evening rotations (and SQ increase frequency) than additional MEB4 rotations. Just a personal preference.
HKG is a good example where it was thought CX would cannibalise from the MEB3 (at that time) and they probably did. However, events subsequent to that show the MEB3 (now MEB4) are stronger than ever.
MAN is, I think, an expanding long haul market. I expect we will see both in time.