It mustn't seem as if they're trying to slavishly recreate the success of Skyfall. An encore by Adele would be much more conspicuous than a second outing by Mendes. I'm afraid it would seem timid, or at least unimaginative.
Quite the opposite in fact.
What's the old saying? "If it ain't broke..."
They would be foolish to ignore the success of Adele's song. Why wouldn't you bring her back? 'Skyfall' was how a Bond song SHOULD sound. Big, loud, plenty of brass and a soulful voice from a fantastic singer. The lyrics weren't up to much but they never are.
Retaining Adele is a no-brainer. Muse will never get the gig.
Sorry, but I can't resist:
"You know that old expression, 'If it ain't broke, don't fix it'? Well, it doesn't work in the film business."
- Michael G. Wilson (source)