Cancer is one of the biggest concerns over extending telomeres length and prolonging 'life'.

If you create cells which will replicate forever and avoid the problems of aging, it may also replicate any mutations which could turn into cancer.

This may create an evolutionary pressure that selects those without the mutations.

Well... The descendants of some of us may become immortal one day.

Unfortunately, evolutionary pressure can't act on a) mutations caused by (foreign) viruses or b) hereditary mutations only expressed late in life, after child-bearing years. Once we've had our children and raised them to be independent, evolution doesn't really care what happens to us and there is little selective pressure to prevent cancer-causing mutations.

There are exceptions to this, but they are few.

You write:

"Once we've had our children and raised them to be independent, evolution doesn't really care what happens to us and there is little selective pressure to prevent cancer-causing mutations."

That is true for some species, but it less true for social animals where grandparents play an important role in delivering resources to grandchildren. Your statement is least true for humans, where grandparents play a very large role in delivering resources to grandchildren.

For our size and weight, humans are unusually long-lived, and probably a reason for that is that grandparents do play a role in the survival of grand children and even great grand-children. Thus, among humans, evolutionary forces are still at work even late in life.

Social behavior and the role of grandparents has created some evolutionary pressure towards longevity beyond childbearing age.

Also, keep in mind immortal humans may want to bear children at ages much later than current ones can. For instance, it would be helpful if you could bear children at 30 year intervals so you could be sure you raised your kids right (or try a different approach if you didn't)

I'm no expert about this topic (effects of society on evolution), but it seems to me like after a certain age, there are diminishing returns from having grandparents. One critical point might be when you get a stable job. After that, the role of grandparents probably can't compete with other selective pressures to keep those grandparents cancer-free. Right?

As to having children later in life, that could certainly help the situation, but only if we all agree to do that. If there is some genetic factor that determines when a person wants to have kids, then people who want to have kids earlier will likely win out.