What do you think cancer is... Cancer is when cells divide rapidly out of control, causing damage to the body.
What do you think youre body is doing at this moment?
All cells need to be replaced. Telomerase just allows the cells to remain healthy afterwards.
Yes you can stop cancer by stopping telomerase, but it isnt without consequences to youre health. It's not going to be a cure either, you cant keep someone on anti-telomerase forever.
With luck it will be an add-on for treatment, not a miracle cure.