Yeah, wow. It was...painful, wasn't it?
I disagree with you about what Johns did, as you know -- he wasn't the problem with the Legion. He tried to fix what was broken long before he touched the property, and while you and I don't agree on the fix, I don't think there's much doubt that the property was critically ill long before "Legion of Three Worlds."
The problem with that statement is that Johns really didn't fix things, and he made some things worse. He ignored the Legion's underlying structural problems (aging team, continuity issues), and managed to make thing worse in some cases. Look at all the continuity issues that cropped up when Johns came back, and were left unanswered by him and Levitz.
Worse, it seemed that Johns was less interested in reestablishing the Legion as a property that could stand on its own, and more of a conduit for characters he seemed to care more about, like Superboy Prime, Conner Kent, and Bart Allen. Lo3W seemed more focused on them than it was on the actual Legion (or Legions, as the case may be.) Look how quick he dumped writing the Legion backups in Adventure after the book was revived.
Paul Levitz set it up for anyone to pick it up in the future, though yeah, there were some brutal endings to some characters in this final arc.
Sadly the damage to the Legion now is so bad that any new book with the "classic" team would feel more like a FYG-type of book. I honestly don't feel Legion works in a dystopian setting like that series was. It works best in a "Star Trek" kind of setting, where Earth and its allies are the good guys, and not a "Babylon 5"-like "us against the universe" setting, so to speak.
But nothing in this industry is forever, so we're just going to have to settle in for a wait, hope that a creator comes in with a good take on the Legion and does well with it, if/when it returns.
I seriously think it's gonna be five years or more before anyone thinks of touching Legion again at DC. Probably not until the next DCU reboot.