I doubt the new engines will be scrapped, money will already have been spent on development of them. And besides don't the new engines better suit the engine manufactures like Mercedes, for purposes of Road cars?
And besides McLaren have to start buying their engines either next year or the year after. And over recent years the Mercedes engine has been the best engine. So it is possible that McLaren might not be able to afford them.
Also the Vodafone deal is coming to an end I believe, unless it's renewed. And with the loss of LH it might have an impact on sponsership deals.