None of the existing chat apps have established themselves as viable alternatives

Meta has trashed their privacy image so FB Messenger/WhatsApp non-starters for lots of Americans. Signal, telegram don't have enough PR, 90% of Americans have never heard of them. Kik was popular but died due to their financial trouble. Discord/Groupme have found success by marketing themselves towards particular niches, but people don't really think of them as general-purpose messaging apps