Rondo is a smart guy.
Why would he negotiate an extension with one team before the trade deadline when he can simply wait until the end of the year and negotiate with all teams.
He can choose the best combination - location, winning team, coach and money ... and he can use the Celtics potential max deal as leverage. He'll have more leverage with more competition for his services. He can also better establish his value by playing out the season. Negotiate now and he's not going to get the best extension.
He knows all this.
IMO no way he agrees to an extension before the end of the season. No extension, no value back to the Celtics. No value back to the Celtics, no trade.
He doesn't want to move for a 3 month rental. He wants max control (because like Ray he's a control freak ... in a good way).
Unless he's a team-killer, there's no way Ainge moves him for low value. He has a lot of value in mentoring the young kids and helping Smart through his rookie season. He helps the development and value of every other player. If he's gone, the team is probably not maximizing the value of everyone else. From his perspective and from the Celtics perspective, the smart move is to play out the year.
By far the likeliest scenario IMO is Rondo plays out the year with the Celtics and tests free-agency next summer.