I recently changed my mind on this: being able to use phone numbers as identifiers also has some benefits: Last time I was on vacation, it was easy to get in touch with service providers (e.g. surf school, massage place) via WhatsApp, simply because I had their number, and everyone used WhatsApp anyways. If Signal would be more popular, I could instead contact them directly via Signal, since I only need their phone number. It allows for a more seamless transition.

Matrix OTOH would have a harder time getting adopted: businesses would need to update their websites, facebook pages and flyers to add the matrix contact info. Which they wouldn't do until Matrix is more popular. Which is a chicken and egg problem.