Yes but do they offer the same experience? Signal figured out that you probably have a list of contacts that you want to talk to. If they use mobile numbers as identifiers then they don't have to keep the contact list - it's already there on your phone. IMHO it's a good compromise.
I'm not an advocate for Signal, but I totally get this approach.
That does not make sense. There is no relation between 'using phone number as id' and 'storing contact list on servers'. E-mail and same other communication protocols also do not use phone numbers and do not store contact list on servers.