Tornadus-T can not be solely responsible for keeping Keldeo's usage down ~_~
Actually, yeah it can. Most average players consider Keldeo a waste nowadays, especially on a rain team, because of the perceived prevalence of Tornadus-T, who is generally an exceptional check to Keldeo. When you take a closer look at the usage stats, you'll see that Tornadus-T is ranked a lowly #19, used on less than a tenth of OU teams. However, he mere thought of Tornadus-T is so threatening that the average player will refuse to use it on the ladder.
I do not believe Tornadus-T's presence is actually enough to make Keldeo not worth using. Keldeo, especially when paired with Terrakion, can be an absolutely devastating Pokemon, with the ability to even bust through your everyday special attacker's greatest fear, Chansey/Blissey. This makes it a fantastic Calm Mind user when combined with its great base Speed and decent bulk. Keldeo can singlehandedly clean through entire teams, and it is a huge threat to the metagame that should not go ignored.