Here's a thought, only based on your quote ... the quote doesn't say when those last words of hers were said...could be 6 months, could be 30 years. I'd guess it's one of those open-ended, whatever you want to believe endings. :whistling:
....but I haven't actually seen her ending slide/carried her to the end of the game, so I probably know nothing. heh
I think you are right about the death happening in later life.
Cass lives to see Hoover Dam in all its Old World glory, humming with power the likes of which the Mojave had never seen. Vegas burns brighter than ever, Securitrons fill the streets, and Cass's heart skips... just a little. Her last words are to the Dam - and to herself. "We were going full speed ahead... but facing backwards the whole time."
"Cass lives to see.."The only question is the amount of time, if it was near the end of her natural lifespan the sight could have made her heart skip a bit too much resulting in a natural death.
However her last words are quite saddening and the fact she is saying the words to the dam implies she might be there to jump after seeing how the Mojave is heading backwards.