it takes time, time in wich u can get killed, meaning if 2 people swing you you are most likely not gonna escape that easily, thats how its supposed to be
nerfing Jett's mobility is stupid, if you're gonna limit her dash you might as well take her out of the game
that said, there should be more options to counterplay her mobility, and I think KAY/O's ult buff is a step forward in that direction