There are numerous examples of BALPA, under a clear mandate from its members, preventing the further degradation of our working practices by opposing schemes such as this. A recent example of a similar situation is TCX pilots successfully preventing Ryan Intl' (US Airline) summer only Captains coming to fly G reg TCX aircraft and thus denying commands to time served guys who have dedicated their careers to TCX. Whilst not the same as the situation you propose the underlying principles are.
The Pilots of the airlines you mention and aspire to are likely to strongly oppose your plan (a la TCX above) and therefore, no, it isn't the employers who ultimately decide and yes, I believe my comments are relevant to the question asked.