Miller's also worth a lot less to the A's unless they convert him back to starter, and they may prefer to rebuild with position players, who have less volatility than pitchers.
That's a big leap of faith for a guy who's only been THIS guy for 13.1 innings in the majors and has already had a UCL issue. And you're still trading a potential middle of the lineup bat for a reliever. Even for an elite reliever, that value proposition will end up very lop-sided if Basallo comes close to what people think he can be.