To be clear, I don't think headscarfs alone will immediately change anything. This is more of a bellwether. A signpost of a country slowly sliding away from secularism and towards what could eventually become theocracy. And that's sad.
I really don't think it would and here come most of people issue to understand politics in muslims countries (and i agree that it's very complex), it's that we always think in term of "who are the islamist" and "who are the seculars" in situation (like in Egypt) when the so-called "secular" are using religion as much as the Muslim Brotherhood (you can look upon the so-called secular Sisi imprisoning atheist and homosexuals). Or in Algeria where the government heavily use religious rhetorics in the 80's and then became a kind of "secular hero" putschists against the democratically elected islamist FIS.
So we apply different criteria to different situation. In Turkey secularism is very specific, it mean the total submission of the religion by the State and to the State. Religion is on the ID, it's in the constitution and it's considered a core-value of the Turkish nation as a cultural trait and not a faith or guidelines. And the army is supposed to enforce that it's stay that way but they used when it was needed religion as well and at the end is what more about securing their own power than "preserving secularism".
The elites were (and still are, in certains areas) the representation of the ideal secular Turks the system would produce and the college education system would produce this same elite and it's was why the veil was banned in the university.
But in parallel to the religion domesticated by the State, clandestine form of religiosity always existed like the Sufi Brotherhood (banned in the 20's) and diverse groups like Milli Gorus and the Gulenists, who aimed to get the power back to the civils and emancipate the religion from the State. This strict control of the religion by the Military "secularist" give some weird situation, like the fact that the so called "Islamist" Erdogan is actually giving more religious liberty to the minorities (several church were builds and some citizens swaps their religion from islam to christianity when it was impossible to do so before, because kemalism made Islam like an ethnic characteristic of the Turks).
So to conclude, for us to witness the birth of a theocracy in the Saudi or Iranian sense of the term, we would need to have the same kind of dynamics that those countries had when they gave birth to the system they are actually living in. That would be armed rebellion against the State with the masses supports and with a clear ideological roadmap. This situation don't exist in Turkey and the AKP is not a ideological party, it's more like a tendency that is supported by a lot of different kind in people in Turkey.
You have the entrepreneurial class who like the "business-friendly" approach, you have the low middle-class who appreciate the kind of "solidarist" reforms like the universal medical care and of course you have the religious minded who see the AKP as a way out from the constant
secularist pressions of the elite and army. And the Kurds as well who would be interested in a "post-nationalist" concept of the turkish nation, and seduced by the promise of a possible autonomy from the AKP.
Now we are 15 year after, and the political landscape of Turkey was totally changed. Of course you have the authoritarian derive of Erdogan, condemned by many early supporter of the AKP, but you also have the end of the military rule and a political class more representative of the people they are ruling for. Some old issue who seems impossible to resolve like the hijab in public space is now a "non-issue" since even the kemalist "secularist" party CHP have a hijabi deputy now and they use this as well to show that they have changed. Erdogan still repeat to this day that he is in favour of secularism, even if some voices in the AKP (the oldschool) are clearly in favour of eliminating secularism of the constitution, but not as a state practice.
Nobody, even the more extreme Saadet Partisi, is speaking about imposing a clerical class who would rule the country like in Iran or in Arabia Saudi or even about shari'a law.
So with this kind of configuration and national dynamics, it's very unlikely that we would see the emergence of a theocracy (in the sense of a ruling clerical class) in Turkey. It would have been possible for instance if the coup was successful, the AKP will have been heavily targeted and will hence radicalize and adopt revolutionary perspective, exactly the same way the Muslim Brotherhood in Egypt radicalized themselves or even Khomeini who was a monarchist in the early 60.
It's also important to stress the fact that the clerical class don't really rule, except in the judiciary, historically in the Sunni world, and the temporal power (Sultan) and the religious power (the Ulemas) were most of the time separated since the Umayyad.
