Selective exposure bias and cognitive dissonance absolutely do not only apply to information (in as you said "debate and 2nd hand reporting"). Their effects are equally documented on first hand experience.
Two people with difference beliefs experience the same thing and each walk away with their previous beliefs reinforced.
I hear and grant your argument ("What I'm suggesting is that where we don't observe significant adaptation in preconceptions within a shorter time scales, we might discover that humans are more capable to adapt over a much longer scale.").
But you're seemingly only using that to suggest that there will be one impact (towards more flexibility / change). You're ignoring the other side of the sword: what if more experience only deepens our beliefs because of our hardwired cognitive biases? E.g. if its difficult to overcome 20 years of racist enculturation, then what about 200?
The worries I have, and why I bet more on my future than yours, is that the majority of these biases stem from non-conscious processes in our brains. So there's no indication that taking a brain adapted for a 30 year human lifespan and extending it to 200 is going to radically change their physical, biological underpinnings.
However, if we gain the technology to indefinitely extend life, it seems probable that we'd also have the technical capability to modify these logical flaws. So it's possible we'd be able to "debias" brains to a greater degree.
Or, and I'd actually consider this more unlikely and harder, perhaps our understanding of human psychology and neuroscience will have advanced to the point where we can consciously override our own biases in a more reliable and passive manner (e.g. super mindfulness, Dune-esque psychoconditioning).
My main point is that indefinitely extending life and expecting our mammilian, product of evolution brains to just "get it" isn't a bet I'd be willing to take with humankind's culture hanging in the balance.