Earlier quoted context omitted.
WhatsApp didn't fade at all. They have 1.2 billion monthly active users and climbing. Their "stories" feature got 177 million daily active users in just 10 weeks. For comparison, Snapchat is only at 166 million DAU as of Q1 this year.
WhatsApp is (AFAIK) the only major chat service that offers a J2ME app. There are a _lot_ of feature phones in the Global South, and a lot of telecoms that charge gouging rates for SMS and MMS.
WhatsApp's support for J2ME ended in 2016[1] - it is now smartphone only, because it requires "advanced features" (or bloat) to compete with Snapchat and complete Facebook's flanking manoeuvre (along with Instagram).