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.
Perez is pretty well connected with Telmex (and Carlos Slim himself), is he not? I'm sure they'll bring in more than enough money to keep things running neat and tidy. Mercedes will still want to sell their engines to other teams, and given how reliable and quick their current engines have been over the years, I'm sure McLaren will still be first in line to buy them. Telmex McLaren Mercedes, anyone?