Smothers Brothers ended two years before Laugh-In aired its pilot episode. So I’m not sure what kind of “competing comedy variety TV shows” you think was happening there.
And, sure, having Nixon on was a dumb thing. As with Trump on SNL it probably handed him the election. But to argue that it was “pathologically centrist” is, I think, to miss the overall bent of the show.
Parts of this remind me of No Soap, Radio which was less controversial, thus not as well-remembered, but still very experimental. And it made it to five full episodes.