We are already at about 30% non-believers in 2018. The lack of death would boast those numbers by at least twice.

Only when the power and money that organized religion aggregates and arrogates to itself begins to wane, will it’s influence do likewise. It exists primarily as an instrument of social control. It won’t release its grip so readily

"Let's imagine there is a breakthrough medical discovery and human age can be extended to 200y+ and even more over time.."
Then you have a different problem
We cannot radically extend all human life and keep increasing population, so either having children becomes a privilege or life extension. Radical life extension would be a privilege at the outset by nature of invention alone, reserved for those with the capital to afford it; something for the wealthy.
Thus the poor would remain faithful, and tendency would be for the wealthy to hoard that tech and leave the poor with faith instead to resolve the population issue.

No hope for mass exodus from religion as people must learn to question,

I said 10% as a guess. I really have no idea.

Between 10-20% not much change-some religion is so engrained in some people. Their lives revolve around their churches.

I think you need to ask yourself what is your purpose in life to live more than 200 years
Is there a purpose

Religion does use death to its advantage. The fear of eternal punishment after death is probably a pretty big motivator for many. If people were to live significantly longer, it doesn’t seem likely that many would give up religion except that they would have more opportunities to change their minds. Since the faith virus is so strong, I’m going with less than 10%.

why guess?

I cannot even guess. I do not think that it is the fear of death that keeps people glued to religion. It is the fear of life.

Whilst I agree that mortality is probably a huge reason for a lot of people, I think this may be hard to judge. As we advance our sciences and technologies as a species (health science included), we see less and less youth prescribing to a religion. So whilst they may happen in unison, it may be hard to pin point it down to life extension or any other particular scientific breakthrough. Of course I could be wrong and it could be the largest reason!

This claim has been made and there is some truth to it. In this case people will still have to face death eventually so I do not know that the fear of death would lessen. The fear of Hell and loosing ones community does seem to also be a huge factor that may actually be a bigger fear than actual death.