Long term wear causing shrinkage or leading to erectile dysfunction is a myth, you have nothing to worry about unless there is an unsafe cage or practice done that causes physical harm. however that's the same risk of wearing jeans that are too tight or playing sports. Like anything else, male chastity is about common sense and most of the stories you will hear about someone being locked up permanently and never let out for months at a time is just a fantasy or pillow talk.
100% of the videos you see online of "husband has been locked for 3 months and finally let out" are completely made up. It's a great fantasy and I love to tease my husband that we will do that, but as with any kink/fetish/whatever shared between two people who respect and care about each other it's all about reality and moderation.