If we assume that Janna has 35-40% CD Reduction:
Howling Gale is a 6 second CD
Zephyr is a 5 second CD
Eye of the Storm is a 6 second CD
Plus, this is not a normal squishy build, taking CD Reduction from a Frozen Heart and Morellos Evil Tome.
Let's say Xin uses Audacious Charge with both Xin and Janna at full health. If Janna sees him coming, odds are, she'll put up her shield. So Xin rushes in, deals damage to the shield and begins his Talon Strike.
Once Janna is knocked up, she casts howling gale. As Xin is right next to her, there is a very low chance of Xin dodging it. This gives Janna a chance to cast the Zephyr slow onto Xin, as Janna moves away from Xin. By this time, her shield is back up, and if Xin is following Janna reapplies it. And now the speed of zephyr passive is reapplied, giving Janna more movement speed. She has the option of keeping the passive, and using another Howling gale to knock Xin up or make him dodge it, or to use zephyrs slow again.
This doesn't cover factors such as boots, rylais crystal scepter, or her ultimate, each which can be used to outrun, push, or slow Xin.
This won't always happen. Sometimes the Xin will kill the Janna. Sometimes the Janna will get away. Sometimes the Janna will kill Xin. Yes, I've seen it happen, and I've done it.
I'm just saying you can't assume that every "support" character will behave the same way as in SR. Support has evolved into some interesting builds that make them more than capable of sustaining and aiding a team, whether in shields and heals, or even flat AD and AP. There was a Sona on our team once (a personal favorite character of mine) who told us she was going AD Critical. I was surprised, but she was insistent that it would work.
It was an incredilbly close game of 10-0 as the final score but we won. And that Sona had captured the most points, defended the most, died the least, and had the 2nd most kills on the team.
What I'm trying to say is that anything is possible, depending on the ability of the players, the item choices, and the composition of the team. Don't be so quick to bash support, when they have quite a few excellent rolesmto play.