Whatsapp's Achilles heel is surely Facebook ownership. While Facebook became a success sort of by accident, everything they touch seems to turn into poop, yet they have the money from the biggest user base out of all social media platforms who are there just because everyone else is, not because Facebook is doing something right. I am afraid that they might fuck Whatsapp up, although technically it is an amazing messenger, and it would be hard to integrate Facebook into it as it was used by masses of people who outright refused to use facebook messenger.
Technically FB Messenger is a heavy and clumsy tool, compared to the tiny Whatsapp that literally just polls once in a while for messages and the only data transfer and processing happens when the messages are sent or received. The battery use spikes are likely correlated with times of having poor internet connection/signal, as Whatsapp would be checking for internet and trying to resend/re-receive complete message until successful. Facebook just gives you an error in such insances saying it can't be sent and would not attempt to receive messages until it "reconnects" (which is also why I prefer Whatsapp).
The future problem will surely be monetization. Facebook makes money on ads, yet Whatsapp is not making any money on itself anymore, as it's free and not ad supported, and Facebook killed the yearly fee, making it free for everyone, yet leaving its business model something to think about in the future. One day that future will come and users might not be happy, depending on what Facebook comes up with.
For now it's great, I feel like Facebook knows that Whatsapp works best due to its technical superiority and simplicity, and they are kind of afraid of touching it, funding it from other areas of their operations. Maybe it will last, or they figure an un-intrusive way? I remember they tried to add a feature last year or so, got boo-ed and stopped touching it ever since, and I guess that's what most of Whatsapp users want. I use it because it's simple, secure, and just works really well in all regards. I'm happy everyone is on it too.
Discord owns and has access to your communication for any reasons, including the right to sell it - I would never use it.
Telegram has no legit encryption. I'm not talking on-device encryption, but it's not a problem to obtain messages on their path or from Telegram servers. It's what Whatsapp was in its early days, except with no user base, and Whatsapp already evolved to be better AND has users. Except Whatsapp never claimed to be something it's not, which is what bothers me with Telegram - they claimed they are safer than other messengers while being amongst the least secure ones using very simple, basically unencrypted communications. If I wanted anyone's Telegram messages, I'd go to Freelancer.com, pay someone 20$ and have it, that's how bad it is.
https://en.wikipedia.org/wiki/Telegram_(messaging_service)#Encryption_scheme
To be fair it's a little better than Whatsapp was before it introduced full end-to-end encryption, where you could get whole conversations as pure text by re-routing traffic from the wi-fi hotspot. I could do that at home and read my girlfriend's messages by logging in to the router from a Linux virtual machine. Now Whatsapp's encryption is top-notch, while Telegram did not move one bit.
To me it doesn't matter as not that many people use Telegram, and I have doubts of it ever competing with established messengers, and I have a feeling it is more likely to be a one-hit wonder that will die out, a little like Gtalk.
Technologically speaking, the crown and userbase belongs to Whatsapp and Line, with Facebook Messenger being a contender in terms of pure userbase, but is technically completely garbage.