This really isn't true either. JBJ is as confrontational as they come, yet he was the first opposition MP elected in the Anson by-election in 1981. Proving that was no fluke, he was re-elected at the 1984 GE.
Of course, you have the more mellow Chiam being elected in 1984 and a relatively more confrontational Low Thia Khiang getting elected in 1991.
Essentially, it's the issue of branding that makes the difference. After all, if this was purely about personality, how the hell would hooligans like Shanmugam remain?
Essentially, if you are looking at regime change, I think you are still a generation away - you just have too many boomers still around who can't be swayed. Just as people lament you have the hardcore opposition supporters who would vote any opposition, you have still a sizeable chunk of those who would vote any monkey PAP fields. Only mortality would help reduce this base.