Snapdragon has only announced their interest, anyone else could still surpass their potential offer. There will be a bidding war for naming rights.
Full naming rights are worth twice as much as partial naming rights like Spotify Camp Nou. It'll depend on how strongly people feel about such things, and how willing the owners are to listen. People will be more open to renaming the new stadium, if Old Trafford remains but partial naming would still be most fans' no.1 choice.
It's also a case of leaving huge amounts of revenue out the door, when rivals like City(esp. with inflated deals), Arsenal and soon Newcastle probably will benefit more from full rights. We'd be worth around £600m for a 10-year deal.