Signal hasn't blocked Beeper throughout the years nor has it limited people not in Signal's hardware ecosystem from joining nor does it have nearly as much market penetration. WhatsApp even did get regulated recently via the DMA. There are plenty of arguments against Beeper's stance on iMessage but "there are other chat apps which exist too and maybe they do maybe they don't have same problems" isn't a particularly strong one.
I'm not making an argument, I just found it humorous to say that a company had a monopoly on one of their own products. Not a category, but a specific product. The whole idea is just absurd.