The deal will leave the seller at your sons mercy for decades (the life of the loan).

What is the point behind having paid off real estate, if you lose it later in life to oppressive taxes?

Insurance right now is wild, so I'd get a solid broker on your team - I'd be happy to share my contact if that would help.Also, aren't most of the existing insurances shared on the OMs for the properties? 

Don't make your life more complicated than it has to be.

But it's still...whole life insurance.

The increased cash flow would make such a substantial difference in your daily life that not paying it off would be foolish.Outside of that, I can't see many reasons I would take a 15% tax hit to save 4% on a mortgage, which is really only effectively about 3-3.5% depending on your tax bracket.

Quote from @Peter Walther: I think you'll find most, if not all title insurers, will require the DIL to be dated and executed subsequent to default in order to insure. 

It's not an insurance company/carrier. 

It's even worse for landlords "The company is also asking for rate hikes of 38% for rental dwellings and 15% for tenants, with the rates taking effect May 1."

Health insurance, 401k, etc., and usually 8am-12/1pm daily.