They aren’t sticky enough on their own to avoid people just installing the next one. I think I have used 10 of the things at various points but SMS and email are still massively more popular. Remember Yahoo messenger etc?
In some countries - primarily Russia, but also in Europe e.g. Germany and increasingly the UK - Telegram is also popular, and Viber is popular in African countries, while WeChat is used extensively in China, and so on... people have to use the messenger everyone else is on.
I do use Signal to chat to close friends, and Telegram has become popular enough as a second to WhatsApp that I often use it too, but as others have said it's a strong network effect.
The only other messenger I can remember having the position WhatsApp has was BBM back in the day. But as soon as BlackBerries went out of fashion it was all WhatsApp and has been ever since.
https://www.statista.com/statistics/271561/number-of-sent-sm...
That works in a different way. Today you have WhatsApp and Signal. 95% of your friends are on WhatsApp and 5% are on Signal. Now let's say Signal starts taking over. In five years it's the other way around. This is possible because you can install them both on the same device during the transition.
If you buy a kind of phone that can run Signal but not WhatsApp at the time when 95% of your friends are still on WhatsApp, that doesn't work. You're not going to buy that phone right now because your friends haven't moved yet and won't for another five years.
First you have to spend five years getting everyone to switch messaging apps, and even then you can only switch platforms if the next messaging app to become popular runs on the new platform, which it generally doesn't, because nobody writes apps for platforms nobody uses and nobody uses platforms with no apps.
The usual solution to this is something like wine that allows you to run the old platform's apps on the new platform. But Google prevents that by encouraging third party apps to have dependencies on SafetyNet or some other Google services or code that the new platform can't easily implement as a result of excessive complexity or legal restrictions.
The effect is like crystallization even if 99.99999% of a mixture is in state X only a tiny seed is needed because it’s a one way conversion. People have a reason to install app Y, and no real need to use app X over app Y but they do have a reason to use app Y over app X.
Meanwhile the first kid could only convince five out of twenty friends to install the new app, and also couldn't run some other apps the new platform doesn't have, and then complains to the parents to return that phone and get a different one.
Further people often switch clients even when they have friends on the old platform simply because it’s the new thing. That’s how all the current major messaging platforms took off, none of them are very old.
People switch to new messengers sometimes, true. But the next one isn't going to be yours unless you have a surplus of luck in addition to skill. The next one is with high probability going to be someone else's.
The real problem is that when it happens, you need the new one to support the new platform. Which it doesn't have any incentive to do when nobody uses the new platform. Convincing the next one to be on your platform is the same problem as convincing the existing one. The transition has nothing to do with it.
Consider even just adding advertisements is enough to eventually lose the top spot because there is so much competition.
I asked google:
number of signal users: 40 million
number of telegram users: 400 million