1. Aljunied GRC is an almost surely a lost cause. The WP has become so entrenched that dislodging them will be a Herculean task. So no point sacrificing Ministers. Remember that George Yeo was a significant loss to the party and the government. This PAP team is expendable.
2. The PAP believes that the WP's reputation and competence has already been torn to shreds so that whoever (read PAP) contest the ward will surely win against the WP. It won't be a walkover, of course not, but the WP won't stand a chance as the PAP steamroller comes to town.
Whichever, my sense is that the PAP does not think that its chance of winning back Aljunied GRC is very high. But then, you never know. The same way that George Yeo was defeated may come back to haunt the WP this time around.
p.s. I stand corrected. WP's founder was the late David Marshall.