The founders aim to keep it free forever, should they want to earn money they'll add some non critical features so people who want to pay will keep the app running
I guess they have several more years to introduce something. About the business model and monetization, the Telegram FAQ [1] has long stated this:
>Q: How are you going to make money out of this?
> We believe in fast and secure messaging that is also 100% free.
> Pavel Durov, who shares our vision, supplied Telegram with a generous donation through his Digital Fortress fund, so we have quite enough money for the time being. If Telegram runs out, we'll invite our users to donate and add non-essential paid options to break even. But making profits will never be a goal for Telegram.
Well, they do not aim to earn money out of Telegram, so I guess that'll just work fine if they sell some stickers or things like that. The thing about Whatsapp and others is that they are a profit minded company, Telegram isn't, they just need money to keep it running