"This is simply untrue, the Bulgarian did dislodge the Byzantines from 90% of the Balkans for generations..."
And of all of the invaders of the Empire, only the Bulgars were able to actually dislodge Byzantine control over settled populations and develop their own state tied not to Constantinople, but to Preslav
And even then, Bulgarian control until the modern era was only centralized around Thrace, the region least tied to Constantinople.
"...and of course Turks in Anatolia."
What you see as "Turk" never amounted to more than 3-4 beys and their entourages, mostly ex-Roman auxiliaries taking advantage of a civil war to occupy the citadels that were the center of control in central Anatolia; a common Turkish identity, defined by
ghaza, never develop until the late Ottoman period. "Turk" was used for most of the Ottoman period to refer to any Muslim in Anatolia & the Balkans, regardless of what language they spoke - and the separation between Muslim Asia and Christian Europe is an astonishingly recent one. As recent as the late 19th c., the heart of Ottoman culture was in Rumelia while the heart of Christian Greek culture was in Asia Minor - it was population exchanges that changed that.
"Where did you come up with this idea?"
There is a reason why the Ottoman Sultan claimed the Roman Emperorship; it was to legitimize his standing in the Balkans, and legitimize the millet system he enforced (wherein each religious community was autonomous and held their own land). This was not an unusual tactic, and even the British would use similar strategies when governing India.
"Except the Avars..."
No they weren't - the Avars were never a serious threat to the Byzantines, and the time that they "came close" was when they had the assistance of a far more formidable state whom the Byzantines themselves viewed as equals (and who were simultaneously overrunning the entirety of Syria & Egypt).
"Also the Slavs permanently changed the ethnic landscape of the Balkans, all Slavic populations today have something like 50% Slavic ancestry from Eastern Europe and most have even more, they were not temporary flukes."
First, in the mediaeval world Europe was (more-or-less) thought of as one single
imperium with local rulers, tied down to a Roman Emperor/Pope (in Constantinople or Rome). What mattered in the mediaeval Balkans was not some idea of ethnicity or dialect, but religion -
all Christian Balkanites were considered Roman, with the only fissure thereof being their way of life (settled or rural). Speaking more directly about the Slavs, as mentioned they took over the least settled (as in urbanized) parts of the Balkans and were never fully independent until the 13th/14th centuries; there was no conflict, per se, between speaking a Slavic language and still identifying as Roman.
"Again where are you coming up with this stuff? The Ottoman conquest was all things considered quite fast, in 150 years they went from a small Beylik to controlling all of the Komnenian Byzantine empire."
Not really - the various states of the Balkans were only firmly pulled in the Ottoman orbit in the 15th/16th centuries, when the various nobilities there came to finally accept Ottoman overlordship after
de facto independence (see: Ottoman conquest of Serbia, Wallacia, Moldavia, Croatia, Bosnia).
"Yeah sure, this is why the Byzantines were unable to remove the Turksfrom the recently lost Central Anatolia , why they lost Southern Italy to the Normans or why the Bulgarian were able to rebel as a coherent group, clearly the empire was not that united. Or why the Byzantines had to rely on crusaders and then lost Cyprus to them."
1. The Sultanate of Rum co-opted the same provincial elites of the Byzantine period, who only lost their power after the fragmentation (by which time the Byzantine Empire had
itself fragmented).
2. Vis-a-vis Italy, the Normans once again co-opted the local elites (whose ties to Constantinople had been decreasing over time) and where the locals were disaffected.
3. Bulgaria, again, was largely an exception - and it's only proving my point when after Basil's 11th c. reconquest he had to rely on the local elites to administer the province. When, a century later, this was reversed the Bulgarian state once again reappeared.
4. Cyprus was lost after the fragmentation of the Empire, and - once again - done so first by a revolt of the provincial elites
(who claimed the local pretender as Emperor), and then by Normans/Templars who (once again) co-opted the local elites in order to rule the Empire.