I don't think you HAVE to go natural in order to maintain and have healthy hair. I just know for ME personally I wanted to go natural mainly because relaxers were thinning my hair out bad.
I have naturally fine-textured/low density hair (it runs in my family) and my hair is very delicate, so having had relaxers since I was 10 years old, it was eventually starting to thin my already fine hair after a certain point. I tried for years (6 years to be exact) to do everything under the sun (ie. reading hair books, joining hair boards, stretching relaxers to 3 - 4 months, trying mild relaxers, making sure to relax ONLY the new growth, lye/no-lye, etc), and while my hair health DID improve tremendously, at the end I was just tired of having hair "anorexia", so I decided to try going natural just to see what would happen. The rest is history.
I think it all depends on each person, what their goals are, and how their hair type is. Personally I think everyone should at least TRY being natural once in their adult life. If it doesn't work for you, then you can always go back. Shoot, I may even go back to relaxers one day. I never know lol
But no, I don't think you HAVE to go natural in order to have healthy hair.