The unfortunate reality too is that average users will never go to the extra trouble of authenticating keys themselves. I'm also more likely to trust a company like Apple or Google with key management than a "trusted third party" (simply because they're bigger companies, with more valuable brands to protect, and resources to throw at the problem).
So, it feels like for the average consumer, a product like iMessage ticks all the necessary boxes. While WhatsApp adding features like the ability to compare your keys via a QR code looks cool, I have to wonder how many users will actually take advantage (and how would they even know if that number or QR code really represents what they think it does?)
Manual authentication via QR (or comparing the digits) works on top of the server-side authentication. It does what iMessage does, but you also have the option to actually compare the fingerprints if you are so inclined.
So, it feels like for the average consumer, a product like iMessage ticks all the necessary boxes. While WhatsApp adding features like the ability to compare your keys via a QR code looks cool, I have to wonder how many users will actually take advantage (and how would they even know if that number or QR code really represents what they think it does?)