Earlier quoted context omitted.
I wonder if they record the necessary analytics data from mining the text, and then delete the actual text. Depending on what data you're trying to mine, you may be able to get what you need on transmission, and then dump the source data in favor of the output of the analytics event.
The chats are end-to-end encrypted, so there is no plain text anyone could "mine"
But, the NSA approach to data collection is basically to vacuum it all up and, if possible, decrypt it later. This has two implications:
1. Metadata. "We kill people based on metadata" isn't a joke, it's a quote from Michael Hayden, ex-executive in both the NSA and CIA. End-to-end encryption doesn't hide who you're talking to or when.
2. It seems unlikely that it will be computationally possible for them to decrypt all traffic, but it would only surprise me a little if AES128 is breakable for high-value targets in the next 20 years: increased computing power, better multi-threading, better cryptanalysis algorithms, maybe quantum computing or some completely unexpected technology; it's hard to say what will come along.
In short: mining encrypted data still matters.