Unfortunately, Airbnb is probably the only way a middle-class family can earn money on investment properties. No one in their right mind would rent a $1,000,000 condo for (1% rule) $10,000.
If you have a million dollar condo in addition to your primary residence then you aren't middle class by any common sense definition of the word middle.
The rule is a residential rental is only worth the risk or has a good return if the property can be rented at about 1% of the value per month or more (e.g. a $100k home should rent for $1k/mo). I generally try to get an annual cap rate of 9% or more (1%/mo less expenses).
Not smaller, just different areas. It's fairly easy to achieve that in the Midwest and larger urban areas in the southeast. I typically got ~0.8%/mo or more.
I'd go so far as to say that if single family residential is your strategy it's better to take the multi-million dollar investment in a single home in the Bay and make one in 5-7 (or more) homes elsewhere. The appreciation is drastically lower but the cap rates are amazing (much higher cash flow) and the risk is spread out. Of course, with that kind of investment it's better to go multi-family or commercial anyway.