What were the effects of the intellectual decline amongst Muslims and the caliphate (khilafah)?

I'd say the intellectual decline amongst the ummah resulted in existential issues becoming accepted, even normalised, as non-critical.    What are critical issues Islam identifies?   They are those that warrant the taking of life because they pose an existential threat to the ummah and her body politic. These include: - abandonment of Islam's laws, institutions or way of life,  - introducing political divisions and borders, - advocating kufr ideas and deens - waging jihad to defend or convey Islam Etc.  Thus we see Abu Bakr (ra) fighting Muslim tribes for zakat or rebellion, Umar (ra) willing to kill an Ansaari after the death of the Prophet (saw) for calling for division, Imam Ali (ra) fighting Muawiyah for the bayah, Salahudeen fighting the Crusaders/Fatimids etc.  Historically, and even today, there was often resistance to a breach of existential issues, but this wavered, allowing for existential issues to embed themselves in the ummah's body - especially towards the end of the Uthmani khilafah.  Some examples to illustrate:  - Allowing for political division and total autonomy. Historically regions separated and the ummah did not fight back or resist this eg Spain, Egypt, Iran etc or 50 plus polities today and the ummah sees little issue, worse, justifies and participates in nationalist politics.  - Not calling for or applying capital punishment to those calling and fighting for for kufr deens eg Ataturk, Young Turks, nationalist rulers etc.  - Failing to resist Muslim rulers implementing kufr laws or systems eg importing and implementing of French civil code, abolishment of shariah, European codes and laws etc.