Abdul Hadi Pasha
Banned
Nothing in history happens in isolation, and this is no different. The Theme system had actually decayed and fallen apart before Manzikert, and that's what made the whole post-Manzikert situation possible. The interior of the Anatolian plateau had been effectively depopualted by the 1070's in comparison to Basil's time, so it was simple and easy for the Turkish settlers who arrived to effect a major demographic shift in the area once they had open access post-Manzikert.
So Manzikert wasn't necessarily itself a turning point, but rather the culmination of a series of events that added up to a turning point. The point when you could say the Empire really stopped growing in power and starte waning was when Basil II died without a clear heir.
Myriokephalon was important because it was basically the destruction of the professional, mobile army the Komnenoi had been pain-stakingly rebuilding for the better part of a century. They didn't have the political ability to rebuild the theme system because he had to keep aristocrats jockeying for former family lands in Asia Minor satisfied, so he had no new land to settle farmer-soldiers on. The Angeloi and their actions also didn't happen in a vaccuum, either. The backlash and against the Latins and the aristocracy was well-deserved and had been coming for a while. It just so happened that Byzantium didn't really have the breathing room to undergo such social upheaval without losing their tenuous grasp over the international situation. If Myriokephalon had been avoided, however, it's possible they would have had the forces to preempt many of the things which started falling apart with the deposition of Manuel's heir.
EDIT: And yes, had the early Palaiologoi been more mindful of their Asian territories, or had a Laskrid been the one to take The City, it would have been perfectly possible for the Byzantines to hold on the Aeagean. Nicaean Asia Minor and the Greek Balkans were some of the most densely settled, economically complex areas of Eastern Europe at the time. The resources of Asia Minor are the single main thing which launched the Ottomans to such heights of glory as they achieved.
Actually the resources of Asia Minor had very little to do with the heights of glory of the Ottomans - it was the resources of the Balkans. Asia Minor was just as depopulated and poor in 1300 as it was in 1100 - probably more so, and the Ottomans controlled the Balkans before they dominated Anatolia.
Anatolia was probably somewhat more important for raising troops, but even that was in the later period.
The problem with the Aegean coastal regions of Asia Minor is that they are largely indefensible against a state on the Anatolian plateau - that's why Greece tried to destroy Turkey after WWI - to secure their possession of the coast. And that's why Manzikert doomed the empire.
As for the turning point, it was the disbanding of the eastern army in the 1050's by emperor what's-his-name. I'm sure it was a Michael or a Constantine of some number. But Basil sure didn't help by failing to provide an heir.