While this sounds good, it is hard to believe that this will be more than a dent in WhatsApp’s user base. WhatsApp is way too ubiquitous, and the whole point of using such a service for most users is to use the one that everyone is using. Unfortunately.
Convincing WhatsApp users to additionally install Signal is hard: they already have SMS for the few people that are not using WhatsApp, now expecting them to install a third app for the same purpose seems ridiculous.
Android mitigates this problem a lot by allowing to make other apps — like Signal — the default SMS/MMS app on the phone. Suddenly people are able to use Signal for SMS/MMS and Signal messages transparently. Signal is smart enough to figure out if the conversation partner is using Signal and enables encryption, video calls and other features. If not, it just falls back to plain old SMS. All in the same app, very convenient for the user!
I don’t really get why the same thing is not possible on iOS? Apple is well known for taking things like privacy and security for its users seriously, and this seems like a low-hanging fruit. So dear Apple, wouldn’t now be a good time to team up with WhatsApp-alternatives like Signal to help the users to make the right choice?