Thoughtful, i disagree. Mainstream orthodoxy makes claim to never have changed and accepts a mesorah from rishonim and achronim with complete adherence to every word of chazal as torah misinai, even the things modern society doesn’t like. It doesn’t seek to reexamine anything or change halacha or hashkafa to fit the times, or make sacrifices to “conserve” the “greater” picture. It lays claim to an unadulterated Judaism, and this encompasses sefardi jews, temani, litvishe, chasidishe, yekkish, hungarian…literally every sect of jews who did not submit yo haskalah. Modern Orthodoxy is the only group among the rest of klal yisroel which does not accept the above, depending on how modern they are versus how Orthodox they are.
So they are not mainstream. They are deviating by their own admission. They believe that humankind jas discovered something of importance not given to us at har sinai; in doing so, they have taken themselves out of the mainstream.
Also in numbers, they are the minority worldwide among Orthodox jews.