If it works awesome if not I work another ~3 years.
I am not saying it's a good idea for most people, just it's more possible for a wider range of people than you might think.
I can speak for myself personally, since I have no generalized data, but I have several _close_ friends earning within the top 5% of the nation. Even _they_ would have trouble parting with 50k on a risky investment. They'd _want_ to invest, but that's not an insignificant amount of money to part with for them.
PS: The other thing that happens to most people is eventually their kids move out of the house. That opens up a lot of possibilities even without a pay bump.
Many of us in the HN community have wealthy friends. I'd wager that few us of could get 20 of our friends to invest $50K in our startup.
For similar friends & family rounds I've been involved in, you'll have one or two people cut $100 to 500k cheques; a few with $25 to 50k cheques, and then some from $5 to 10k.