depens how good of a leader alt justinian II is
constantine IV migth still die in 680 or 690s if we look at his art
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the man looked like a living corpse assuming constans dies in 670s or early 680s
he is leaving constantine IV with this .
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and worst case he dies in just 10 years after his dad then even then the byzantine empire is left way better than the otl and alt Justinian II would be left a quite a good place its also likely that the bulgarians would loose ongal was a close thing that the byzantines would have won with out constantine IV leaving constantine IV or an ageing constans II would win that battle and the first bulgarian empire doesnt exist or they move to the north of the danube maybe to panonia.