As with so many things in life, there's truth on both sides. Yes, TOS was an enduring success and that's why TNG was able to be made in the first place; but it was always seen by the general public as something of a cult franchise, popular but not entirely respectable outside the fan community. TNG did bring in a whole new, very large audience and gained ongoing critical acclaim and award nominations, so it did serve to give Trek a wider mainstream legitimacy than it had had before.
Let me put it this way. When I was in high school in the early '80s, being a Trek fan was something that made you part of the nerd/outcast community, something you'd hesitate to admit to if you wanted to be popular. When I went to my high school reunion 20 years later and said I wrote Trek novels, I got nothing but "Wow, that's so cool!" in response. Star Trek absolutely has more public legitimacy now than it did before TNG existed.
Correct. And people today tend to forget just how big Star Trek was circa 1993/1994 as well. I don't think there has been another time, before or since, when Star Trek was as popular with a broad 'mainstream' audience as it was then. And that was totally down to TNG.
Not to disparage it... but to blunt, TOS was never a success. It limped through three television seasons before finally meeting its inevitable fate, and even the movies, as successful as some of them might have been, travelled a rocky path where they never quite broke through that barrier of being 'cult'. TNG is the show that finally proved that Star Trek wasn't just a cult thing. I still meet people today who don't define themselves as "Trekkies", but admit they were weekly viewers of TNG back in the day.
It does get undervalued in that sense.