I volunteer at an animal sanctuary.
I've known about a dozen blind animals there in 10 years, most being canine and fairly old.
One they tell me is blind, but the fact that I carried him to a new location and set him where he'd never been, and he ran around every obstacle, tells me they aren't totally correct.
As for betas. Mine of 3 years I had to kill. He lost the ability to float or swim, which means he'd drown (they're practically air-breathers.) After watching him struggle greatly..... I did a mercy killing and buried him in the garden beneath my window.
It's nice to look through the posts in this thread and see people knowing about their pets-- lifespans, what is going wrong, etc. 20 years ago, many people would not.