Sorry, the argument "the opposition wouldn't be doing this if they were not dumb" is a totally bogus argument. We know that it is possible for the opposition to be dumb. It is like World War I where all the leaders of the world went into war, and every one thought it was the right thing, even though it turned out to be the stupidest thing possible.
The non-WP opposition should not be thinking, "we will fix WP". Nobody likes to see the opposition think this way. Even Chee Soon Juan understands this which is why his proposal was made in the name of "opposition unity". But the reason why his proposal was such a lousy one is because everybody can blatantly see that he's lying.
All the opposition parties roughly know how well they'll do in a straight fight against the PAP. But none of them other than SDA know how well they'll do in a 3 corner fight. If they do well then it will be not be so bad for WP, because people will sympathise with WP. If they do badly, then it will be bad for them, because it will show that they are weak. During the next GE there is still room for all the opposition parties because not everybody who challenged the PAP in 2011 are quality candidates. If SDP pulls out it will be good for the SDP because everybody else will commit suicide and they won't.