It makes perfect sense regardless of funding source.

You want to have some kind of impact, and you end up having to spending money to do that. You want the most impact and your funding is limited.

"Return on investment" has nothing to do with spending

It absolutely can do. You're trying to force a very narrow view of a simple but broadly applicable concept.

I'm suggesting that in the long run, any organization that is hierarchically "overseen" by humans with outsize power compared to the employees (aka a "Board" of special people that are better and smarter and more politically well placed than disposable employees), will inevitably exploit the structure for selfish gain with a probability of 1.

The STRUCTURE is wrong is the point.

The entire concept is built around would-be-aristocrats (Board) coercing the management and employees into allocating property (Money) based on their whim with no accountability or democratic function. It is built to exploit.

Instead they should organize as a non-stock cooperative so that is effectively impossible to exploit. That's the actual answer.

But you should want an ROI as a non-profit. If you spend $1000 on advertising to get people to write their Senator to help push for/against a bill, you want to make sure you're spending it on the most effective way. If it gets spent on mail ads and no one calls, then that is largely a waste of money (regardless of funding model and org structure). If you don't have a measure, you can't really know if money is being wasted. You can guess and have a gut feel, but not scientific, actionable data to change how the org is working to best further the non-profit's cause.

Maybe you can call it something else, but for a non-profit ROI is just answering the question "Are we spending our money wisely?

Or were you referring to the non-profit having a board?

Yes, you absolutely want a board and accountability on how the money is spent.

I trust a non-profit "board" about as much as a slave court judge

https://www.joangarry.com/dysfunctional-boards/

A board (no need for scare quotes) is a legal requirement for tax exempt (501c3) status. And not sure what point you're making with the linked article which is about (relatively mild) board dysfunction, not scamming or illegal behavior.

Presumably the board could be made of every single employee of the non-profit (perhaps with varying voting power depending on factors such as seniority or experience)?

"ROI" just means "return on investment". It doesn't automatically mean that return is monetary. A nonprofit getting results in terms of accomplishing their purpose is getting good ROI.

Most definitions of ROI refer to monetary return. That's why it's typically a percentage.

Using concepts like ROI for non-profit operations is one of the signs how everything is made to emulate business.