I would disagree, to an extent. Janna is a safer play champion. She has always been strong because safe play is always viable. Right now she is creeping on back because Riot has gone out of their way to reward safe play more than ever before (tier 2 tower shields wutttt). She makes it easy to safely take objectives, and since that is this year's focus - ta da! I would go as far as to say that this change for Janna, the people running towards her thing, is actually a buff for people who tend to play her more reserved; it gives her even more power and rewards for playing safe!
The break down in Thresh vs. Janna comes down to play-styles. No one ever really goes "THIS GAME WE SHOULD PLAY UBER SAFE AND JUST WIN SLOWLY", everyone's got to get that first blood and make plays by level 2. Janna, being a safe pick, can work in almost any team so long as they play it back. Thresh, on the other hand, instigates early aggression and rewards you for it (not that he can't be played safe, but come on a skill that literally pulls enemies into you?). At the end of the day Janna is the better and stronger support, even if she sees little play during meta shifts, while Thresh, on the other hand, is a one trick pony who makes plays and is played frequently.
Anyway, my two cents as a support player - though only Platinum.
tl;dr:
Safe play is always in season, whereas aggressive play is higher risk higher reward. Therefore, Janna > Thresh on average.