1. npubs literally take less than a millisecond to generate
2. tor allows an attacker to keep changing their IP every minute or two, defeating IP based block/rate limiting
3. wasting resources recognising images with an AI engine is costing money
4. it is an assumption without any basis that this attacker wants to pay to make this happen, and if they do, then there is per-subscription limits, and the option that abuse can cancel the subscription

bitcoin survives because it pays people to protect it, subsciption services survive by charging people to use them