Earlier quoted context omitted.
Why are people downvoting this? While the "public key" is not "WhatsApp's" it is served from their server hence in theory they can provide you with any public key they want, decrypt the message, store it, and re-encrypt it with the "correct" private key and send it off to the user. With PKI the ability of the user to verify that they received and used the correct public key is critical and while I have to admit that…
That's true, but they provide means to verify the fingerprint of the other party, so you can verify that your app is encrypting messages using legitimate public key and that there's no MITM going on.
How foolproof is the verification system, how susceptible is it to downgrade attacks (while E2EE isn't not universally deployed) is there are 3d party verification of signatures, is there a community trust signing, can whats app disable E2EE in it's application without a noticeable UX change to either party, how does this work with multi user messages, how does this work with multiple devices, how does this work with historic messages that were encrypted using different keys etc.
I would say that there are sufficient "unknowns" at this point to take the security of this entire solution with some skepticism especially if you remotely planning to use this for anything that could put your life at any risk.