Explain to me what policies McCain advocates for that are so incompatible with the Dem party platform that it can come even close to being a "diametrical" opposition?
Is it her desire to police uteruses? That's not an issue the dems have historically cared to do anything about in spite of myriad opportunities to protect the right to control your own body. She'd hardly be the only anti-choice democrat.
She changed her mind on opposition to same-sex marriage. She's helped pass gun control packages. The only thing she's been vocal about is being anti-Trump, and the best way to defeat Trump is not as Republican and is CERTAINLY not as an independent actor. Hell, running third party will probably increase Trump's odds.