I think you're not wrong about self-esteem improving from doing things that are productive however this is not the case for people of dealing with an actual mental illness. My dad was a very productive person but that didn't stop him from taking his own life earlier this year. I truly think if he would talk to somebody about his anxiety issues it would have helped him get through it.I went to therapy right after it happened and it helped me immensely. with that said I still do agree with what you say and that most people probably don't need therapy, they need to find something that gives them purpose.  

 Sorry to hear about your dad brother