There's about 500,000 h1b holders at any given time. That's less than 0.5% of the workforce. Probably a higher percentage paying into Social Security who will die prior to being eligible.

According to this article, about 20% die prior to age 65, and some percentage of those will not have a spouse to provide benefits to:

http://home.bt.com/lifestyle/wellbeing/did-you-know-20-of-me...