I think in this case, willingness and ability end up in the same place. 

Bitcoin fees are paid from the inputs, so either you have enough or you don't. 

If it's not economically worth it to you to make the transaction (and pay the fees) it's the same as not having enough to pay the fees. 

As far as the fiat to bitcoin to shitcoin to fiat cycle, no one can mint bitcoin, so shitcoins (including fiat) valued in bitcoin will continue to decrease. As long as they have to buy bitcoin at some point in the cycle (which they do in order to pay fees) they will run out of money unless there is a real economic use for what they are doing. 

Willingness to pay is subjective. That's what you said. Not ability, willingness. 

>If bitcoin fees were paid in fiat

Use fiat to buy bitcoin to mint shitcoins to sell for fiat. Repeat.

random_

I think we may be going in circles, but I do think that the economic truth of being able to pay transaction fees or not is about as objective as we are going to get. 

If bitcoin fees were paid in fiat, I would agree with you, but they are paid in bitcoin and so, yes, I think the ability to pay them is an objective measure of whether something is spam or not.

Scoresby

bitcoin

Opinion: Spam can't cause high fees - Thriller post

> I think the only definition of spam that doesn't bring in subjective value judgments is what I said in the article: transactions that aren't willing to pay the fees.

Yea... that's not subjective at all. 

/s

> But what if someone uses bitcoin script in a novel way, kinda like BitVM? Is the script exogenous data?

Depends on the locking script. Probably. 
