Oh wow, more ramble.
I believe Kokonut explained about how SS works - in 1950's, 15 workers pay SS to 1 retiree but today, only 3 workers pay SS to 1 retiree.
Unlike 401k, you don't completely pay SS tax into system and if you contributed about $50,000 to SS, so you decided to receive SSDI because you have disability and the monthly payment set about $1,500 per month. If you have SSDI for 5 years - $1,500 x 12 (months) = $18,000 x 5 (years) = $90,000. It means you robbed $40,000 ($90,000 - $50,000) from taxpayers.