Backdoors are something that spying agencies create with anti terrorism purposes or geopolitical interests - such as spying to key people that could become a threat to the country. It's licit for them to do this, but by doing this they open the door to other hackers discovering these vulnerabilities.
Durov even affirms that Whatsapp has never been safe. In 2012, when he started working in Telegram, Whatsapp sent all messages in plain text, allowing for government, hackers, operators or WiFi network admins could read the messages.
Despite the cyphering added after Telegram was launched, Durov affirms that this was all a marketing idea, because the key in decyphering the messages is in the hands of foreign governments such as Russia.
This is funny, because despite the cyphering, Whatsapp started to insist to the users to create copies of their Whatsapp messages and store them in Google Drive with one of their updated functionalities. Being these not cyphered and in plain text (without Whatsapp saying anything about this).
With this, the FBI could - for example and via a warrant- order google to hand in any possible information from any individual that they deemed necessary. And even if you decide not to store these conversations, they could still find out by asking other people who had conversations with you.
What Durov is suggesting is that Whatsapp has not been safe at all intendedly. And this has been such that Whatsapp founders long time back left the company because - according to their words - "The app was selling their user's privacy".
Of course, Durov is suggesting the people to switch to Telegram (his own app) from Whatsapp. What is really intriguing is the fact that despite these claims - and even if they were real - and despite the "efforts" that Whatsapp has made to make itself more secure and less vulnerable, user privacy does not seem to be a demanding issue that users care for. Despite the fact that a user can know that Whatsapp is insecure, they will still sacrifice their own privacy in exchange for a convenient and free way of contacting their friends.
4 years ago, Edward Snowden allready showed up in Last Week Tonight regarding all of the government surveillance scandal that envolved Google and the NSA. When Snowden (back then exiled in Russia) revealed that the NSA was passing on nacked pictures of people, he suggested this was not a big deal for the users.
If you had to ask us, we'd suggest to never share private sensitive information via Whatsapp.