Ratings merely capture a tiny part of the total audience. Shows are rerun, shown in foreign countries, there are DVD sales and so on.
If we followed your ratings logic and applied it consistently TOS, ENT and Firefly would suck big time. After the commercial failure of TOS, the self-fulfilling prophecy of the third season (if you want a show just neglect it business-wise and it will fail) and the subsequent cancellation Trek fans should really know better than repeat such nonsense about how ratings and business success reveal anything about the quality of art ad infinitum.
About your other point, I have watched Trek for more than 10 years until I actually watched TOS (i.e. really watched it, not some random and badly into German translated episodes). So it looks like Trek can very well be enjoyed and be successful (I bought quite some stuff in these 10 years) without any familiarity with the first show.
This holds for all show, you do e.g. not need to have watched TNG to watch DS9 or VOY.