I think it will be difficult for Norwegian to displace TCX from their position at MAN. It is obviously not impossible, but there is no obvious "extra" or material price discount that Norwegian can offer above TCX.
Additionally, Norwegian continue to post large losses. This can be put down to any number of factors, and can of course be covered by investors. However, it does not suggest that Norwegians current model is a runaway success or that it is repeatable in any number of markets.
The only thing Norwegians presence would do at MAN would be to encourage IAG to respond and create a race for the bottom. This may prejudice VS and UA in the short run, and undermine the market in the mid to long term.
AA/BA will undoubtedly be back (assuming they leave at all) but in the meantime, opportunity for TCX, VS and UA to grab market share.