How people would act? The masses would never change their way of thinking, meaning there would be no progress / change of culture as well as the danger of a bad culture persisting for millenias. At the same time you would see those in power conentrate even more power and it would be certainy power like you see in non-democratic countries, where one individual rules everyone, except that individual would never ever die. Imagine if Stalin has lived to this day. Now considering that, you would think that a 1984-like goverment with a supreme leader that lives for centuries would let the average person live forever in the first place?
> versus guaranteed doom
I would not call the death, complete absence of everything, following a life well lived, doom. Everything has an end. Of course if that happens too soon, that's a tragedy.
> No more passage of time, no more conscious thoughts or experiences, ever.
It's interesting to look at the opposite. If following death, there's no experience, no awareness, nothing. Then what is the now? what it means to be alive right now, to perceive and how is it different from the memories of the past and ideas of the future? It may sound far off, but in a way, we die every moment. And on a larger scale, the 8 y/o me, the 16 y/o me is no more, not in just terms of being made of different atoms, but in terms of identity. So you can also imagine that if a person lives thousands of years, they either experience many "deaths" over time or they live the same dull life for thousands of years - in which there was no point to live that long in the first place. I can share that fear though, but it would be more in the context of "you have x months / years" to live heard at a relatively young age with life feeling far from finished, but the longer I live and the more things I do and experience that I wanted to do and experience, I imagine the easier it is to come to terms and be grateful for what has happened already.
Don't have anything against what they are doing and heck, if I could easily afford it, maybe I would even sign up - one way ticket to see what the future would be like (vs. nothing like you said). But I don't think it's that important and if humanity reverse engineers the brain so that a person can live for thousands of year, or potentially infenitely (I think that would be the biggest challenge that goes far beyond just stopping death, I would not expect a brain to work for hundreds of years without serious modifications), if everyone has an access to it, it's like one of those things like an omni-intelligent AI, something you would think everyone would be better off if it was never developed.
I think it would be cool if aging was stopped (the prime years of adulthood are so short) but I'd wish there was some natural cap of say, 200 years max were the brain would just break down at the end and it would be very hard to make it work longer).