Did the stress of the defeat impair Manuel's health? Because if so you could argue that without the defeat at Myriokephalon Manuel lives a few more years and thus Alexios II is of age when he comes to the throne and there's no regency of his unpopular Empress.
Though even if Alexios is an adult when he comes to the throne he could easily be incompetent, his mother could still cause trouble, he might be confronted by structural issues (as mentioned above) and preyed upon by rapacious cousins, so...