This seems like a private money/hard money situation and they are trying to be careful and anticipating how the market will react. 

Wait and see game to see how the virus and economy reacts in next 4-6 months.

I think when you have a serious problem, how you react to it identifies what kind of landlord you are.  

This I'm advocating is the wrong response.Instead landlords should take a deep breath and allow the stress hormones and neurotransmitters in their body to subside and then make a rational win/win strategy.  

As a property manager we try to get into the properties around every 6-9 months (more often if we think the tenants are going to be a problem) - we can tell by the way they react to rent due and work orders they put in. 

They have more manpower(bodies) and are doing it every day so they are gonna be able to work faster. 

Depending on their personality, they can react from one extreme to the other. 

Unless the existing system is near a body of water and pollutes a lot.Maybe connecting to the existing sewer would be cheaper anyways assuming that you can and its economically feasible.

evidence by definition: "the available body of facts or information indicating whether a belief or proposition is true or valid"

You still just have to ask and then react to the sellers response.