AIs can become at least as smart as human beings, that we could live throughout the solar system, and that death by old age is probably unnecessary
That is much better starting point than the original post. Let's explore those lower bounds:
Even if we can't make something smarter than us, there's no reason to think we can't make something faster than us. Plus, it's only a program, so it can duplicate itself, say, over the internet.
Even if it's no smarter than us, its still can be pretty much unstoppable.
The sun is due to die some billions years in the future. Hominoidea only exist for 28 millions years, so, our potential is at least 2 orders of magnitude longer than our past. Add population size into the mix, (let's say, 10 billions and stabilizing, while our past would be more like less that 1 million for most of the time), and we add 4 more orders of magnitude to the value of the future.
So, the potential of our future is at least a million times greater than our entire past. That's one hell of a potential, for a conservative estimate.
I don't have good "mainstream" for the feasibility of long lifespans. Like wedrifid, Id' just avoid the subject if possible.
Now we can speculate about reasonable sounding worst-case and best-case scenarios:
Best case: Machines take over chores that make us unhappy. Lifespan is about 100 years, healthy lifespan is about 60-80 years. This goes on for the few billions years the Sun let us have. Not a paradise, but still much better than the current state of affairs.
Worst case: Skynet wants to maximize paperclips, we all die very soon. Or we don't do AI at all, and we kill ourselves in another manner, very soon.
There. Does that sound mainstream-friendly enough?
The project of Friendly AI would benefit from being approached in a much more down-to-earth way. Discourse about the subject seems to be dominated by a set of possibilities which are given far too much credence:
Add up all of that, and you have a great recipe for enjoyable irrelevance. Negate every single one of those ideas, and you have an alternative set of working assumptions that are still consistent with the idea that Friendly AI matters, and which are much more suited to practical success:
The simplest reason to care about Friendly AI is that we are going to be coexisting with AI, and so we should want it to be something we can live with. I don't see that anything important would be lost by strongly foregrounding the second set of assumptions, and treating the first set of possibilities just as possibilities, rather than as the working hypothesis about reality.
[Earlier posts on related themes: practical FAI, FAI without "outsourcing".]