From my understanding S40 does not support any sort of multi-tasking and in order to use whatsapp you have to open the app every time you wanna see the notifications. Unless whatsapp comes pre-installed as a system app. I could be wrong on this though.
You reap what you sow. This is what happens when you take away choice from consumers. Even with 88% split going to developers, they can't recoup costs. Meanwhile, Helldivers 2 is a massive hit for Sony thanks in big part to Steam.