Now how about if you take that $20k/year in down payments and instead buy two properties per year with just 10k down each. Also, pull out all of your equity each year and buy more houses.
Let's also assume you're good at cutting corners, so expenses are only $335/mo instead of $500/mo. Payments will be $465, so that's $200 cash flow.
year1:
2 houses, worth total $200k.
$4800 cash flow
$11,160 equity
year2:
2 year1 houses, worth total $200k
3 year2 houses, worth total $300k
total = 5 houses
equity = $11,160 - $10,000 (downpayment for 5th house) + $27,900 (equity earned on 5 houses in year 2) = $29,060
cash = $4800 + $12,000 = $16,800
year3:
2 year1 houses = $200k
3 year2 houses = $300k
6 year3 houses = $600k
total = 11 houses
equity = $29,060 - $30,000 + $61,380 = $60,440
cash = $16,800 - $10,000 + $26,400 = $33,200
year4:
...
11 new houses = $1,100k
total = 22 houses
equity = $60,440 - $60,000 + $122,760 = $123,200
cash = $33,200 - $30,000 + $52,800 = $56,000
year5:
19 new houses == $1,900k
total = 41 houses
equity = $123,200 - $120,000 + $228,780 = $231,980
cash = $56,000 - $50,000 + $98,400 = $104,400
year6:
35 new houses == $3,500k
total = 76 houses
equity = $231,980 - $230,000 + $424,080 = $426,060
cash = $104,400 - $100,000 + $182,400 = $186,800
year7:
62 new houses = $6,200k
138 total houses
equity = $426,060 - $420,000 + $770,040 = $776,100
cash = $186,800 - $180,000 + $331,200 = $338,000
year8:
112 new houses = $11,200k
250 total houses = $25,000k
equity = $776,100 - $770,000 + $1,395,000 = $1,401,100
cash = $338,000 - $330,000 + $600,000 = $608,000
year9:
202 new houses = $20,200k
452 total houses = $45,200k
equity = $1,401,100 - $1,400,000 + $2,522,160 = $2,523,260
cash = $608,000 - $600,000 + $1,084,800 = $1,092,800
year10:
363 new houses = $36,300k
815 total houses = $81,500k
equity = $2,523,260 - $2,520,000 + $4,547,700 = $4,550,960
cash = $1,092,800 - $1,090,000 + $1,956,000 = $1,958,800
So after 10 long years, you have ~$6.5 million in equity & cash. You'll also earn an additional $6.5 million equity & cash every year, assuming you keep it at 815 houses. The houses you own are worth $81.5 million, not including appreciation. Not bad for only investing $20k/year.
But... is it feasible?