I have fine hair, but I think of medium density. I used to wonder about whether thicker hair strands/density was necessary for long hair, since it seems those with thick hair have a lot more hair to work with when they experience breakage or damage. Also, those with coarse hair can take more and are less likely to break. I doubt it now.
Us fine/thin-haired ladies have it hard. But just the same as everyone else, its all about retention--we just have to learn how to properly retain our type of hair. Henna definitely helps make the hair look fuller (by coating and thickening each individual strand) and medium protein is now a must for me. My hair generally looks thicker and feels coarser/stronger now. For those with fine and porous hair, ACV or lemon rinses are also important to close the cuticles and help them lay flat, which protects the hair strand.
Those with coarse hair have to DC or baggy constantly, so they do have their own battles.
The right styles are important too. Twist-outs and braid-outs, rollersets, shrunken twists are all good and they're styles many thick-haired women can't pull off without it being too much, especially without a huge headache to deal with while detangling.
It will be even harder with relaxed hair though, but you should really get a hold on rollersets, pin/spiral curls and the like. My hair was extremely stringy and thin when I was relaxed, but it looks completely different (much fuller/thicker) as a flat-ironed natural. Still, back then rollersets and bumping did the trick.