If it's simply cash flow--irrespective of hard assets-- then you're better off buying some form of debt.

Personal background: Married with no kids, 100% debt free, live a rather tame lifestyle and don't spend large amounts of money, two newer vehicles (2020 & 2024) so no vehicle purchases expected for the next 10 years. 

My buy box is a home 5 years or newer, A or B-rated schools in the 200K-400K range that will appreciate over time with someone paying down my debt.

They show some creative ways to borrow money or to payoff debt using credit lines but their disclaimer is to see your cpa and/or lawyer.

If this is done, then 75% of the gross rent for your current primary can be used to offset the mortgage debt of that property to 0.

If I go the multi-unit route I would put myself in debt for a bit, which is fine for long term investment.

The deals I underwrite have been cash flowing right up until I put debt service on them, even with taking a break even amount in a refi. 

Ownership is different than who's in debt. 

If you don't have the downpayment, go make some cash so that you do, either through saving from your normal income or some real estate investing efforts.Your point on debt paydown, although true, is not relevant to the argument of BRRR vs. traditional rental.

So now instead of feeding the hungry stock market you can feed the hungry rental.I have watched people keep big debt on their home and then have the stock market crash.