There's no need need to sue unless you've somehow been damaged. Dropping service and keeping your 600 dollar phone or returning it used is more than damaging enough for them.
Should I switch to the carrier I left because they have no coverage? The one that's going bankrupt? The other one that's probably going bankrupt tomorrow that tried to merge with them? Or maybe one of the MVNOs that's really just one of the above?
There isn't much choice available.