Not only because of the safety, but because the list of available companies that insure knob & tube is tiny and because they are taking on the 1800's technology risk, they are likely going to charge you for it.The upgrade will likely cost a bit but your place will be more marketable and also, all in all, safer.  

@Alberto Nikodimov what systems/technology do you use to help you manage that many STRs?

Technology today makes it so easy to look at all the markets and find what you are looking for.

What Unique Platforms and Technologies do you Offer?

Have any credit card processing, technology,  or marketing tips?

I have no problem with that, although typically they are not the best on the technology end.

I looked into apartment buildings to buy or build (using prefab technology), or should I go with buying residential homes in different markets?

I like some parts of technological automation, but I'd like to at least talk to the tenant and screen a little before I show it to them or let them apply. 

Technology these days .. 

There is a convergence of people being willing to live in smaller/alternative living situations (new generation) and the technology to make it happen (smart homes are here but alternative building techniques have been slow to catch on due to outdates building codes- but the technology is there) I think we are going to see some massive shifts in the next 20 years.