(This is one of those pop culture facts I didn't realize until recently, as I never liked the original series, but adore Next Generation. All of its cultural impact was from just three seasons, and possibly most of its immediate impact from the first.)
It was actually cancelled after its second season but a huge fan reaction persuaded the executives to renew it for a third, which was probably a mistake as that was by far the weakest season of the show.
I struggled with TOS since I watched TNG/DS9/VOY first. But then I started to enjoy it. It is still enjoyable for those retrofuturistic vibes, reminds me very much of classics like Time Machine or Forbidden Planet.
(This is one of those pop culture facts I didn't realize until recently, as I never liked the original series, but adore Next Generation. All of its cultural impact was from just three seasons, and possibly most of its immediate impact from the first.)