I'm not sure that we have the same definition of 'suggest'...
If we're lucky, we'll only get actual evidence after many decades, like with Crypto AG.
(But of course we already have some evidence, for instance about ECHELON and PRISM (Skype...))
If we're unlucky, black hats (criminals and/or anti-West countries) are going to lay their hands on those backdoors first, like with the NSA/Shadow Brokers leak :
If we're lucky, we'll only get actual evidence after many decades, like with Crypto AG.
(But of course we already have some evidence, for instance about ECHELON and PRISM (Skype...))
If we're unlucky, black hats (criminals and/or anti-West countries) are going to lay their hands on those backdoors first, like with the NSA/Shadow Brokers leak :
https://news.ycombinator.com/item?id=12302085