While you make a nice argument for pain not being a unit of effort, I think a subtlety is ignored—pain at the threshold of current capability is not differentiated from pain derived by trying to perform well beyond capability. The 20th rep of a 30 lb dumbbell curl hurts. But the pain doesn’t endure. You have a meal, get some rest and recover. However, you risk injuring yourself if you attempt the same with a 100 lb dumbbell. While the former contributes to muscle growth, the latter causes injury. Both are painful, but the pain is different. Pain is crucial for growth as it is an indicator of performing at your current threshold. Hence, pain is indeed a measure of effort, but it doesn’t scale with effort. More pain does not mean you’re doing better, but lack of pain could be an indication that you’re taking it too easy.
While you make a nice argument for pain not being a unit of effort, I think a subtlety is ignored—pain at the threshold of current capability is not differentiated from pain derived by trying to perform well beyond capability. The 20th rep of a 30 lb dumbbell curl hurts. But the pain doesn’t endure. You have a meal, get some rest and recover. However, you risk injuring yourself if you attempt the same with a 100 lb dumbbell. While the former contributes to muscle growth, the latter causes injury. Both are painful, but the pain is different. Pain is crucial for growth as it is an indicator of performing at your current threshold. Hence, pain is indeed a measure of effort, but it doesn’t scale with effort. More pain does not mean you’re doing better, but lack of pain could be an indication that you’re taking it too easy.