I think we are within grasping range of genetic engineering humans to eliminate a lot of the more “interesting” bits of biology, but we are waiting for the commercial requirements to be there before really making things happen.

Space stations need spin gravity for long duration use.

As far as living on other planets goes: there is gravity. What we don't know is how much gravity we need. We could answer this with a space station with spin gravity by trying different rotational velocities and monitoring the health effects.

The Moon has about 1/6 Earth gravity. Is that enough? We don't know. Mars has a little over 1/3.

Long duration space flight probably needs spin gravity too. There are two ways to do it. One is a torus shaped spacecraft. The other is two spacecraft and a tether where once they insert themselves into their cruise transfer orbit they link, extend the tether, and start orbiting each other in a spin configuration. People could go between the ships if necessary by following the tether, though this would probably only be done when strictly necessary due to the risks.

There's a calculator online for spin gravity based on what we currently know about human tolerance for the Coriolis effect:

https://www.artificial-gravity.com/sw/SpinCalc/

It's not too bad. There are various workable sweet spots. Playing around I find one at 20 meter radius and 0.66g (2/3) gravity. It looks like you can't get much smaller than 15 meters diameter without ill effects, but we actually don't know. We need to try this to see what humans can actually adjust to.

The simplest test short of a centrifugal station would be to put two capsules or Starships in orbit and tether them and experiment with different tether lengths and rotational velocities.

Edit: actually there's a third way to do gravity on long duration flights. If you watched/read The Expanse you saw it. Accelerate to the midpoint, flip, decelerate to the destination. Unfortunately that requires insane physicsts' nightmare propulsion systems like the fusion engines in The Expanse that we do not have and won't have for the foreseeable future unless there are some crazy fusion breakthroughs.

Why not just use rotation to generate artificial gravity via centrifugal force?

I guess the answer could be that the sideways motion will drive you crazy due to coriolis forces? See 6 minutes in https://www.youtube.com/watch?v=nxeMoaxUpWk

But they can just substitute windows with monitors that compensate for the view.

Incidentally, the viewscreens on Star Trek etc. always had the ship right side up, so they probably compensated exactly in this way :)

The way I see it, it’s a matter of creating suitable environments for humans in space. For the most part, that’s perfectly achievable even with current technology; the primary bottleneck is quantity of building materials, as human-friendly habitats are by necessity large and complex (space for centrifugal gravity, plus room to move around for mental health, plus space for all the requisite air process facilities and such).

So this is another area where bringing down cost of pound to orbit is highly impactful. If that becomes cheap, it becomes feasible to build in orbit crafts (orbital stations, ships, etc) sufficiently large to support human-friendly habitats. Should Starship+Superheavy succeed, it will be a significant step forward in this regard since it can put the interior volume of the ISS in orbit in a single launch, and perhaps in a few decades use of resources mined from asteroids could become a consideration.