Even though I was here on Steem pretty late compared to many other witnesses I'm from "the old school", so in my humble opinion, witnesses are here to serve the platform.
So while looking just at ROI is a valid approach from the business point of view, I think it is potentially harmful for the platform. Because what if the price of Steem would go down to levels that we had in March? That would make every $100 to turn into $1,20.

So if one thinks about earning on Steem platform, then the answer is simple:
"Blogging is the new mining." ^(TM)
Easier, faster and more scalable.

PS
Hardware is cheap but I don't think $40 is enough to run reliable witness node these days, not to mention witness infrastructure.
(More on that in future episodes)