All of these apps seem to hate open platforms and third party clients; Signal just as much as WhatsApp. I wouldn't even mind using WhatsApp if I could just open a browser window on any modern computer and log on like I can with Twitter. But no, I need to have a smartphone with either Android or IOS. They all want that magic unique personal identifier that is the mobile phone number to prevent you from having more than one persona, and they all want their closed apps as the sole way of using their service.
Of course, that requirement is exactly how they implement the user lock-in, so it's not going anywhere until legislation forces them to open up.