Competition is good, especially with regards to privacy and cyber security. Customers benefit when companies don’t have a captured market. A lack of competition only leads to monopolies and stagnation.
If no one offers a secure product, then customers have no choice but to either not use anything or put up with it. Competition means that at any time a newcomer can offer a better product (that is hopefully open source).
Edit to add: Ideally you could message people on different apps with the same account. But I’ll take fragmentation over a monopoly.