Norwegian has an exclusively long haul fleet of 787s therefore the issue is of much more significance to BA which has 767, 747 and A380s.
You can speculate wildly if you wish but the available facts are that IAG has kept all airlines that it has bought running as separate entities (with the one and only exception of BMI which got merged into BA). Not only it has kept them separate but it has grown their operations wherever possible.
The model is well understood and used by many business, for example VW. It runs VW, Audi, Skoda, Lamborghini and Bentley. Why run separate brands? because it captures different consumers with different preferences while still taking advantage of economies of scale. If IAG buys Norwegian and shuts it down the only thing that will happen is that a new player will fill the void