Because John_Smith total reliance on intra company seniority is stupid and has disappeared in the rest of the world of industry some time ago. To expect a senior captain to slot in at the bottom of a seniority list of expectant first officers is a ridiculous expectation. And by the way, "loads" of monarch captains is an exaggeration. It will be a handful. The numbers quoted earlier in this thread were there to wind up some participants.
This whole issue needs to be put into perspective. Inter company movement of pilots is healthy for both airlines and pilots. To effectively eliminate it except in the event of company failure is bonkers and not in the best interests of anyone. What you are saying is effectively you are stuck with the company you join after CTC for ever unless you are prepared to go to the bottom and start again. Does not happen anywhere else and it is madness that the concept still exists in airlines. Seniority lists have been sold to you as a great idea by the airlines at this juncture because it suits them at the moment to drive down costs. If there was free competition (like the rest of the business world) then maybe we would not be seeing such an erosion.