Except people at one point realized that IE is shit and switched to better alternatives. I don't see this happening with WhatsApp - everyone and their great grandfather uses it and all other messengers are niche occurrences
Reading the links provided would have answered your question.
Provisions for ensuring a higher degree of data portability, interoperability, and access to data for the platform's business and end-users.
There is some discussion on if this means that 3rd party apps can develop for chat platforms, and it seems so at face value. eg, gaim/pidgin or trillian can make a come-back and you can use whatever client you want to access the Whatsapp platform. Nothing would then stop an app from natively integrating whatsapps apis alongside their own.
Oh my sweet summer child. You're underestimating Facebook's capability to use shady tactics to keep the users on their platform even if such law is passed.