Telomerase doesnt cause cancer. It allows cells to divide perpetually. You have telomerase expressed in testicles to prevent youre babymakers from horribly mutating.
Also in early development all cells express it, and even later in life there remain many cells types that express telomerase. Just not much, but that little bit is needed.
In fact, the more would be even better as it allows cells to divide jsut as rapid as in young children, so you have a better immune system and wound regeneration.
Telomerase allows cancercells to divide also, but blocking out all telomerase in the body is not wise, you need to block it where it needs to be blocked and nowhere else.
Why else did you think humans carried a telomerase gene if it doesnt do something good.
What do you think cancer is... Cancer is when cells divide rapidly out of control, causing damage to the body.