Even if the jets went after manning they should retain Sanchez unless they get a good offer. (This of course depends on the contract terms of manning.) We might jut find that after a year or so of learning from a good mentor vs learning from a washed up act like Brunnel that Sanchez might come out better than ever.
If there wasn't a salary cap, I would consider it. I would probably still deal Sanchez since if you assume Manning is your guy for the next at least 3 years, you've pretty much wasted half of Sanchez's prime as a backup. Nevermind, he would almost definitely leave when his contract is up to start elsewhere so the idea of Sanchez being your guy Post-Manning doesn't seem realistic in any scenario.
But it's a moot point because you can't have a backup QB making 10 million a year under a salary cap. You just can't.