Sites are always getting new visitors, losing old ones and the ones they’ve observed return irregularly (or commonly, or somewhere in between). So it’s not realistic to assume a given sample of visitors is the population.
It's not unreasonable to assume it's a sample, I just don't think it's worth getting paralyzed by worrying about whether or not you have power, or getting into hacky tricks to try to fix it.
...but most power calculations are also sort of bullshit.