As a thought exercise, how would one even begin to try to pollute a curl with ads? Would it print out suggested websites after every get request?

Probably. That's basically how npm ads work.

Oh if we’re thinking of terrible ideas, it could save ads as a JPG in to the folder where you saved whatever you were grabbing with curl.

Or even more terrible, open the image full screen with no way to close it until it feels it has been open long enough to close on its own. maybe show some sort of timer counting down, and then before dismissing itself, it opens the App Store listing for the app. it'll be very convenient for the user as no user interaction will be required for any of this

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Detect iterm or kitty or other image-capable terminals and display an image

That is definitly an idea, see npm packages.

sell advertising space on the progress bar and charge for faster download speed

I mean, there are a a number of tty graphics protocols. I’m sure with enough dedication someone could figure something out.

Careful what you suggest! VCs are starting to back open source companies these days!