I beg to disagree. SCO itself comprised of three major civilisations - China, Russia and CAR. Turkey is culturally linked to CAR, not Middle East, so it's quite natural for them to be seamlessly merged with SCO. In fact even China and Russia are quite plural, say many Chinese non-Han ethnic groups are Muslim, Mongolian and of CAR heritage.
China and Turkey might have differences, however these are not vital to survival of both nations in the long, more dialogue will help, and SCO maybe a good platform to start with.
Turkey is not culturally linked to the Central Asia as strongly as imagined. A travel across the major Turkish cities like Istanbul will defy the perception of its being a West Asian nation, let alone Central Asian. A lot seem to have changed in Turkey, along with culture over the past several decades.
They can still argue very early historical roots, but, this means little. Siberians may also claim historical roots with American Indians. But, what do they have in common? Besides, since when ethno-culturalism became a benchmark for the SCO membership status?
Being seamlessly merged into the region would mostly mean rising ethno-sectarianism, which does not bode well for the region. Perhaps the most striking difference is the secularism inherent in the culture in East Asia, which spreads though the Central Asia by means of institutions like the SCO, business and people to people contact.
Definitely, China and Russia have various groups of peoples; but, they are part of the larger social fabric, in most cases. The moment they are not capable of it is the time we have problems.
I agree that China would not dislodge Turkey altogether, or any other country, for that matter. But, it also cannot make a long-term partner with Turkey. In my view, the SCO membership would be a very aggressive, radical move. Perhaps, before that, Turkey needs to be socialized into less comprehensive frameworks and be assessed in terms of performance.
Turkey's sectarian interventionism in its south is a very concrete evidence for strategic analysts and decision makers; they are real. One cannot simply brush that aside and pretend that Turkey inside the SCO would be a different actor. They need to be vetted outside the SCO framework, first. Then, slowly be socialized into the more Asian frameworks. Otherwise, the SCO will be much weaker, not stronger, because, currently Turkey represents the three dangers that the SCO was founded to fight against, in the first place.
Accepting Turkey in its present capacity would be a direct confrontation with the SCO's very fundamentals.
I do not argue about a contentious relationship. I propose a realistic approach.