People have successfully adapted to all kinds of unexpected habitats here on Earth. Isn't your argument the same as someone from 70,000 years ago saying "Africa is our only abode"?
There are peoples like the Sherpas who live in high places where others can't breathe easily. Apparently their ancestors genetically adapted to these habitats [0].
>H2O to drink There are peoples like the Bedouin whose whole way of living is based on dealing with the scarcity of water in the desert.
>enjoy 1G gravity
Well, you got me here. There are apparently slight differences on Earth, ranging from ~9.76 to ~9.83 m/s^2, but yes, other planets would be much different.
Anyway, I'm not saying it won't be very difficult for people to adapt, but I just don't see a good reason to say that extraterrestrial life won't ever be commonplace.
[0] https://en.wikipedia.org/wiki/High-altitude_adaptation_in_hu...