You can have an increase in population and still have a low demand for rentals or a low enough demand that rental rates do not outpace T&I for awhile.

I will say that during that time, the HELOC interest rates were very low.  

Low entry costs How did you find this deal and how did you negotiate it?

That’s why you get a discounted interest rate and a lower downpayment, because owne occupant properties are low risk. 

With 20 properties, you’d build a nice, diversified income stream over time.Why Choose It: Low entry barrier, easier to pivot if a deal goes sideways, and you can scale at your own pace.Challenges: It’s time-intensive—sourcing 20 good deals takes hustle, and managing scattered properties (even with a system) can feel like herding cats.

And, just maybe you’ve purchased at a point where Detroit has hit its low, bottomed out, and is set to reverse trend and grow.  

It is not low cost.. you need 7-8 rooms per house to make it decently profitable. 

From our experience with clients, you’d typically expect to pay around $1,000–$1,500 for a single-property return, depending on your situation.If that $6K includes monthly bookkeeping and accounting, it could be reasonable; it could also be low, or high... it all really depends on the number of transactions, bank/credit card accounts, and the complexity of your finances.

I have heard about low cost granite / stone, i just don't know where to get it. 

Anytime you have literally 100k lots of record that can be built on and price points so low your going to get a run on the market which if there is a coordinated sales effort it can outstrip the ability to execute..