Signal is still nowhere close to primetime. It’s not just the lack of users, Signal still has lots of day-to-day bugs, lack of features, and it falls behind in ease of use.
All you have to do is go to Signal forums, github, and reddit to see the huge amount of user complaints, many of which have been around for several years with no working solutions in sight. WhatsApp on the other hand works near flawlessly across all devices.
On Signal, you STILL can’t properly transfer message histories between Android and iOS and there are no reliable message export options which are essential features for the masses.
You can’t even backup your messages at all on iOS whereas WhatsApp has encrypted cloud backups with Google drive/icloud and WhatsApp lets you move your message history between the two OS’s if you change phones.
Signal also routinely has problems with notifications on both android and ios. And from a usability standpoint (not necessarily a privacy and security standpoint), the desktop version of WhatsApp is far superior.
Of course Signal is better for ultimate privacy, but apart from general Zuckerberg distrust, the privacy sacrifices you make using WhatsApp instead of Signal, like not encrypting message metadata, shouldn’t be a big concern for most normal people who are just messaging the same friends and family anyway. The chat content themselves are still encrypted, which is what most peopple care about. WhatsApp is also far better than other popular services that have inferior encryption or have no encryption at all.
For high risk individuals or high threat model conversations in which you need to leave no record of a conversation occurring, which is when metadata is the most relevant, then use Signal/SimpleX/similar. That’s what they’re good for. It’s fine to compartamentalize and use both WhatsApp and these other messengers for different use cases.
But until Signal improves tremendously, there’s absolutely no way I’m telling grandma to delete WhatsApp and move to Signal if she’s already comfortable using WhatsApp. This would be a disservice both to grandma and the limited signal developers who will be inundated with even more usability complaints.