I'm not even sure we can extrapolate that the NSA is storing anything like every text message long-term (as opposed to metadata, or samples, or the full transcripts of individuals targeted for investigation, or a machine learning trained weights set to flag messages that indicate someone should go on the targeted-for-investigation list).
6 billion text messages are sent per day in the US. That's about the volume of Google web searches, and I know from experience Google doesn't have the capacity to log every search or the logs of evaluation of every search. If Google lacks the capacity, I suspect the NSA lacks the capacity.
Google absolutely does have the capacity to log every search query, and does - and I say this as someone who has worked with that dataset, if only for training purposes.
6 billion SMS messages, at a max length of 160 characters, is 1 terrabyte of raw text. I think that the NSA has the cash to shell out $100 for a new 1TB hard drive every day... (not even including compression, of which it is highly, highly compressible)
6 billion text messages are sent per day in the US. That's about the volume of Google web searches, and I know from experience Google doesn't have the capacity to log every search or the logs of evaluation of every search. If Google lacks the capacity, I suspect the NSA lacks the capacity.