For Mars, rotation will be needed and it's an easy solution to maintain human health and strength. I use the word easy because no proof of concept is necessary. The $ cost is not considered. And when it is, just remember it's taxpayer's money. We all know the concern here. Or lack thereof.
But there is another problem which has not been solved.....shielding from radiation. We are really naked out there. And so far, we have gambled with probability. But for Mars, we need more confidence and surety.
It will cost lots of money to SAFELY go to Mars. I don't believe it to be cost effective......the only result will be a boost in man's reputation. Mars has nothing we need. And knowledge is vastly overrated.
Until our tech advances, only probes should be sent to other planets. How many probes could we send out for the cost of one Mars manned trip?
If I were running NASA, I would spend much more on hardware, than exploration. We need much faster crafts for real exploration. Velocity and the lack of, is the first problem. And I believe that the math narratives about acceleration and velocity can be nullified. With intermittent acceleration.
For real space travel, we need to do what all other explorers have done. We need to learn to live off the land, off of the environment that we are in. And for space, that means large collectors to harvest particles. Possibly tack and sail on the solar wind. Very large structures. This wind is very fast.