Only a tiny fraction of my contacts use Signal, and most of those are also on Whatsapp, Telegram, Discord, and others.
Signal offers essentially nothing to me.
WhatsApp instead makes tons of money from this kind of metadata.
Many of my family also dropped Signal.
It is now really only used by the hyper-privacy conscious.
Signal is an awesome project but some of their decisions annoy many users. E.g. Signal does not allow to automatically save all pictures in the gallery. It's a privacy feature, but it's inconvenient since it forces me remember to download each image seperately.
I hope that they make it so you can register WITHOUT a phone number. Perfectly fine if it's not the default. This is post is currently implying that is not currently the case.