@quadratur
I understand what you mean but when people were using it for tooltips they were not describing images in the correct way. It created a lot of confusion as to the purpose of alt texts. I don't think I could go as far as calling it a feature ;)
As you rightly point out, title had this purpose.
I agree completely with your observation about exclusive content. I love having access to all alt text visibly here in my Fediverse instance. I'll be making exactly this point in my talk later today.
It's a huge shortcoming in browsers that it's so difficult to view for most people.