Just saw a link posted by a redditor, pointing to this video:
It appears that Ting has worked something out with T-Mobile, and will continue T-Mo service at least through 2020. I'm kind of surprised I hadn't seen this earlier......
Reddit: r/ting | u/comintel-db:
"Up and running on the Red Network (Verizon) Beta"
Yeeeep. Three of the four major networks for the foreseeable future. It's kind of exciting.