This is very subjective preferences and there's no wrong answer here.What I do not like about paying off cash property is that there's no better treatment for tax purpose.Also if I'm still on my early productive age, I would rather have 3 rentals with each 75% LTV, rather than having 1 paid-off primary.The reason is, in 10 years time I would make more money if I invested to several highly-appreciaed house, rather than only one.I am also one who see mortgage is not a debt instrument, but capital asset for less-risk higher appreciated investment 

They can access better financing generally especailly financial instruments, have the scale, and its pretty much completely passive to you.Its certainly much easier to invest via a REIT into various commercial spaces, especially also when the investment isnt' near you. 

I view a HELOC as a great short term debt instrument to get you into deals - like a flip, BRRRR, or a deal that you can't pass up while waiting to sell another property. 

(i) The term "Insured" also includes(A) successors to the Title of the Insured by operation of law as distinguished from purchase, including heirs,devisees, survivors, personal representatives, or next of kin;(B) successors to an Insured by dissolution, merger, consolidation, distribution, or reorganization;(C) successors to an Insured by its conversion to another kind of Entity;(D) a grantee of an Insured under a deed delivered without payment of actual valuable considerationconveying the Title(1) if the stock, shares, memberships, or other equity interests of the grantee are wholly-owned by thenamed Insured,(2) if the grantee wholly owns the named Insured,(3) if the grantee is wholly-owned by an affiliated Entity of the named Insured, provided the affiliatedEntity and the named Insured are both wholly-owned by the same person or Entity, or(4) if the grantee is a trustee or beneficiary of a trust created by a written instrument established by theInsured named in Schedule A for estate planning purposes.

It is a mortgage instrument whereby the S A is a substitute for interest. 

Or using the stock market in anyway ie trading every day or swing trading, misc. instruments etc?

Paying contractors quickly is instrumental to them being on your side.

Many believe this is a cost saving mecanism but in many cases it will cost as much if not more and will not be as sound of a legal instrument.

I’m sure you will inspire others and be instrumental in their success as well.  

As a community we need to work to protect this financial instrument.