I don't think anyone is saying Undergrounds prices are unreasonably high. They are probably spot on with the amount of work and time it takes.
Some people, like myself, get a lot more satisfaction and pride from doing work like this themselves. I've watched several videos on rolling fenders, and it seems like something I could easily do IF I had a roller. Granted, I own a 13 year old car so if I messed up, it wouldn't be that big of a deal to me.
There are some things that would just take too much time and money to learn and DIY. Paint and bodywork is one of those for myself.
Does this mean that Underground will lose business? Probably not. There will ALWAYS be someone paying the professionals to do the work either because they can't do it themselves or because they don't trust themselves with that type of work.