I beg to differ. The nature of parties is that you need to climb up the ranks and in order to do that you need to get into good books with the leadership. It's the same for any party, PAP or WP. Obviously it is not in TJS case. You're talking about SDP here and not NSP or WP who would value TJS credentials more. There's a chance he can't fit in.
Gomez is a newbie but John Tan is not. He came in many years back and would be a powerful person within SDP as ASG. At this point of time TJS would have been appointed as a cadre and if he was, it would give him something less to resign to go for something that he cannot qualify. Also, given his age, it is not likely for CSJ to give way to him. He's nearly 10 years older than CSJ.
He may not realise it, but being rejected by the PEC doesn't put your credibility in good standing. It doesn't matter if the PEC is PAP controlled or PAP affilated because it gives the impression of being a reject. This is a Singapore psyche. SDP stalwarts also do not welcome that because they think the president role is a farce. He is unlikely to be seen in a good light (not in bad light either) if he returns to SDP. And I kind of don't think he will. Many people I spoke to does think it is his exit strategy.