There is one strategic but cynical reading of this news:
AF has followed a sneaky but smart strategy with its unions.

It "sold" TO to unions as non-competing with AF mainline, but competing with other LCCs. It recruited lots of staff under "competitive" contracts. Transavia France grew 12% last year compared to a small growth for the mainline. And now that its operating margin is positive, it is brought under the umbrella of AF.
Sure, TO has a different business model, but 55 routes will be added to the AF network, sold on AF website (probably at prices higher than on TO website as benefits are greater) and provide similar FB benefits. Not sure that AF unions are happy with that. But it is a smart move.