That is where my heart truly has always been. The spin-offs feel like filler.
But does everyone feel that way? I don't question that at least one of the spin-offs was mediocre, but I do wonder if such a thing precludes another series. Revisionist history is a potent commodity. People who bash the spin-offs forget that the third season of Star Trek
was also mediocre aside from a few memorable episodes ("The Enterprise Incident," "The Tholian Web"). The perception of TOS has changed with time because people are more fond of the characters that made up that series.
I'm more fond of Kirk and Spock and always have been. I'm sure nostalgia plays a part in it but it isn't the only reason.
With Berman Trek, we got increasing dull characters, characters who frequently looked down on other cultures because they weren't like humans (I still shake my head over Picard's reprimand of Worf in regards to Duras) and a really odd morality promoting that it was better to allow entire civilizations to die in some odd idea that it's better that they die than see someone more advanced and that because your explorers it gives you a right to trample over others borders.
In TOS, we saw characters that learned that many times their first read on a situation wasn't the right one and they learned from it. During the Berman-era, it seemed like we were the only cultured species out there and we were out there to teach everyone how great we are.
For me, the Berman-era series have aged poorly. I'm not everyone nor do I profess to be. But we already have six-hundred hours of "other Trek" and less than a hundred of Kirk and Spock. I'm one that thinks the core of Trek needs to be explored again. YMMV.