I think it really depends on the individuals hair, and how you take care of it.
When I was relaxing my hair, i could get it swinging and shiny and looking like it grew out of my head shiny and straight, and was quite long. In my opinion I found it dry, and though even at its worst it was longer and healthier than a lot of crispy heads out there-- I know from experience relaxed hair can be quite beautiful and healthy.
Why I stopped relaxing was because I can get my hair just as straight without a relaxer AND I have the flexibility to get my curls back which I actually prefer to straight hair, it wasn't to get it better looking per se. I am also lazy, and think relaxed hair is too damn delicate for me to bother with, and there is too much running from the rain and so on for an active girl who likes to go into tropical jungles and remote areas without steady electricity. I am also scared of strong chemicals like that, and don't need any crazy cancer from whatever gets dumped on my head every 2-3 months. Afterall, your skin absorbs whatever you put on it and that is fact. Not saying perms definitively cause cancer, just saying I'm not taking that chance.
Really I think what really makes people have that stiff and fried look is a lack of moisture, tons of petroleum based products, abuse of heat styling, and general misinformation about how to take care of hair, and in particular black hair. I have seen crispy looking natural heads, and quite honestly i think its a shame.
Healthy beautiful hair is possible with relaxed and natural hair, as is crispy dry scary hair!
Now as for the "white girl flow"
..... I'm just going to reiterate what other people say by pointing out that all white girls do not have the flow. Its more the "hair model flow" ( which is usually a wig or extensions btw).
I have a lot of white friends who's hair does not really flow and who struggle with dryness and breakage (some of them work in hair salons!). If I had a penny for everytime I heard them complain about not being able to grow their hair out I would be hiring and firing bill gates every other day for fun. They over color, over heat, their hair sticks out ( some quite stiffly) and NOT ONE of them has hair longer than neck length. These are fully white, european and american of european decent girls. One of thems hair is so awful me and my best friend refer to it as looking like a chewed up ripped Barbie. You know-- the one your neice cut the hair off and looks like its been through hell and back?
And then they lie and say " I'm just gonna grow my hair out next year, I decided to cut it again" (Oh is that why your hair has been chin length for the last 5 years and you sometimes whine that you miss having long hair? and ask my how my hair grows so fast?)
The white people I know with long healthy swinging hair, don't fry and dye their hair to death. Usually they leave it alone and/or keep it in a pony.
Somebody needs to go undercover and tape this stuff and break the " white girl swang" myth. They do not have the monopoly on nice hair.