I have been privacy aware and a free and open source supporter long before the Librem 5 existed. I have been using Signal since the TextSecure times. All my regular day to day messaging I’m doing via Signal, where I have hundreds of contacts. Also I still have an old Android phone that is the primary Signal device. On the Librem 5 I use Signal desktop, which works good enough for me, but could definitely be better. But also Signal desktop on the Librem 5 became way more usable over time. In general I use Signal on my Fedora laptop or Fedora desktop, which is most convenient for me. So, in one way, you could say Signal still traps me into having an old Android phone as primary device. On the other hand, if Signal was in no way available on the Librem 5, I think I would not be able to use the Librem 5 as my daily driver. I asked many people to install Signal for communication with me. I would not want to ask them now to switch to something else.
I think for many privacy and security aware people, switching to Signal was one of the first things they did on a path towards more privacy.
Also note that Signal is working towards Signal Desktop being a primary device.
Having said this, I think it is very good that Purism also provides the option to communicate via Signal with the company. So they can support people moving towards Purism devices that run PureOS.
I also have the impression that Signal is becoming a bit more (mobile) Linux friendly. There is now also an official ARM64 Signal deb package and AppImage in beta testing.
It’s important to understand that anti-interdiction does not stop interdiction from occurring. The goal is to stop undetected interdiction i.e. to detect it. If you want to stop interdiction then you should drive to California and pick up your order.
That of course only addresses interdiction in a single step in the process i.e. the delivery of the device from Purism to you. Your enemies can always interdict the device after you have taken delivery but that is for you to manage, having regard to who your enemies are, and what techniques and resources they have available to them.
Here’s the next alternative communication app: https://carrierpidge.app/ (I like the idea, not the platform) 
Not strange at all. All ‘normies’ I onboarded to Matrix have since abandoned it due to the key exchange and login verification issues.
Signal is not the best on terms of freedom I agree, but who knows the alternative may have been people expecting to be contacted on WhatsApp or legacy unencrypted calls/SMS.
I see some people commenting on and expecting support on Purism’s Facebook page, so Signal is not really the devil here, just a useful minor demon if my analogy is correct.
This would be great once it arrives. Looking forward to a hassle free Signal setup on Librem 5.
If I understand Signal’s phone number requirement correctly, you just need it to register the account and then remember/save it. You can discard it after this and not need to pay for it every month. If someone tries to register using that number, there’s a 7 days lockout period during which if you open the Signal app, the person who has your old number now cannot register it with Signal. Haven’t tested this though.