When can I have a compatible FOSS desktop client? I don't do 100% of my communications from my phone, and I never will.
If you can't do it on desktop, you can't do it at all. Mainly because some of us have real work to do.
If the only "usable" implementation is on a hard-to-physically-secure mobile device that uses a tonne of different uncontrolled network access points a day -- that's not really an option now, is it?
I feel you're missing open whisper systems' target audience. If you've seen how regular people use computers, their phone is the most secure device a normal person owns. Not the most secure if you're worried about targeted attack, but the best place to put a dent in cheap dragnet surveillance :)
You won’t have that until someone takes the time to produce such a client.
Its like saying notepad is a popular because its installed with Windows.
Even if you ignore Whatsapp and Cyanogenmod, you have 500,000 - 1,000,000 Textsecure installs according to the Play Store. That is on order of magnitude more than the GPG estimate of 50,000. In less than 20 years.
The Axolotl protocol that was created for Whisper System's TextSecure is now used, by default, by Cyanogenmod (10 million users) and the Android version of WhatsApp (more than 500 million installs from the play store).
I'd say Moxie's tech has been pretty widely adopted.