Actually, what you are saying is just not true. I believe that you really believe this, but that's sad because you've been lied to.
First of all, on life expectancy. Amazingly, the designers of SS took that into account! Yes, they knew that life expectancy would rise, isn't that something? This is no big shocking emergency. This is not even a problem. And life expectancy is going to rise to 100 in 2050, really? For whom Onceler? Let's look at how life expectancy has risen thus far:
http://krugman.blogs.nytimes.com/2010/08/13/live-long-and-prosper-2/
Yeah, most of us aren't going to live to be 100.
As far as SS being unsustainable that is just poppycock! I am for lifting the cap, period. However, if that makes too many people clutch their pearls in the worry that when they reach millionaire status they'll have to pay more into SS OMG!, then we can do this: gradually lift the cap beginning in 2030 from 100 to 250 thousand, and raise the payroll tax 1 lousy percent across the board. This will insure the solvency of SS for...well forever.
I don't scare on SS, because the scare tactics are so easily debunked. I'm really surprised you believe them.