As I've heard it, the alternative that drove the first hundred million users was the telco reading over your shoulder and/or randomly dropping your messages on the floor.

Whatsapp grew big with customers of telcos like Vodafone, which may be less capable than Mark Zuckerberg's snoopshop but I wouldn't bet on their being nicer. See https://toroid.org/vodafone-smtp-mitm for example.