Hi, to be gentle and brief: Lengthening telomeres can provoke cancerous behavior. Therefore we cannot simply lengthen telomeres by giving people more telomerase; we need a more holistic approach which understands the cancer/senescence tradeoff.
It is not a problem for a creature to live for a long time; the inevitability of cancer seems to itself be genetic and part of the human experience but not for all life. You mention whales, but lobsters are even more interesting: They do manufacture telomerase throughout their lives, and they are not killed by cancer in old age, but by being unable to molt and continue growing. Trees are interesting too; they must always grow in order to keep living, but past a certain size, the physics of water limits their ability to grow.
Indeed, if we want to understand trees and whales, my first guideline would be that, because they are so large, the rules for cellular homeostasis are different at that scale. The things which allow us or lobsters to live for long times are not the things which allow whales or trees to live for long times.
If you believe that telomere shortening is the issue, we should be investigating therapies to encourage telomere lengthening.
We have access to powerful technologies (e.g. CRISPR), and we can develop technologies that are more powerful still.