I think it’s a valid argument. Once this is deployed everywhere, people will see the statements “Signal supports MOB” and “Signal has 20 million users” and make a conclusion, “20 million people can use MOB”. The only way to avoid being part of this statistic is to avoid the app.

In general, boycotting the product is the time-tested way to send messages to big corp. if you don’t like your clothes being made by child labor, don’t buy the brand. If you don’t like WhatsApp privacy policy, don’t install it. If you don’t like P&G spying on people in China, don’t buy their products. And if you don’t like cryptocurrencies, don’t use stuff from companies which support them.