The Titanic was supposed to be unsinkable too. IMHO there is no level of encryption that can't be cracked given enough time and desire. Right now I would say this type of communication would be very secure, but to think it will always stay secure would be foolish.
True, but... the more people who routinely encrypt their communications, the more likely they won't bother trying to decrypt any particular one.
There's always the possibility that the gov procured some mega power computer that they would be willing to have run for days/weeks/months just to crack one particular encryption. It's also possible they got hold of some mathematician was able to invent/discover a way of cracking codes with a simple algorythm, which would be an "I found the holy grail" achievement. If you can easily find the only 2 very, very large prime numbers, which when multiplied together get a much bigger number, without doing lots of trial and error divisions, then that would be you.
Probably the biggest potential vunerability though is a computer or phone that's been hacked with some kind of spyware. Encryption wouldn't mean squat then.
The temporal encryption schemes are the best. That's what we used in a video conferencing project. The infrequent 'I' frames are a total refresh, but the intermediate frames are not. As the video progresses, the encryption key changes over time; what you may lose in efficiency you gain in security. Always tradeoffs.