Perhaps the answer is that even with the reading μολπᾶν ἀλκάν, the sentence is somewhat odd, and Page didn’t feel that reading was more plausible than his own conjecture.
West and Sommerstein set μολπᾶν ἀλκάν (oxytone) off by commas. Sommerstein’s translation: "for the age that was born with me still inspires me divinely with persuasion, the singer’s prowess . . . " The apposition of ἀλκάν to πειθώ is strange and awkward.
Fraenkel doesn’t set μολπᾶν ἀλκὰν (barytone) off. His translation: "for still from the gods the age that has grown with me breathes down upon me persuasiveness of song to be my warlike strength . . . " He treats μολπᾶν as depending on πειθώ. Again, ἀλκὰν is in apposition to πειθώ.
So there are two somewhat different interpretations of μολπᾶν ἀλκαν, both of which seem somewhat awkward, even though some marginal sense can be made of them. And σύμφυτος αἰών compounds the awkwardness and strangeness. What does that have to do with the underlying idea? It can be explained, of course, but it still leaves me (at least) wondering. So both of these solutions, if plausible, are not entirely satisfactory, or at least Page apparently didn’t find them so.
Page probably felt that σύμφυτος requires a dative. σύμφυτος with what?
But I can’t say I find Page’s conjecture wholly satisfying, either. However, he uses daggers to indicate his uncertainty about the text, and he offers his conjecture as a possibility, not a certainty, whereas the other editors apparently feel confident that their readings are correct.
We shouldn’t fall into the error of assuming that a ms. reading that presents difficulties of interpretation should necessarily be preferred to a conjecture, just as we shouldn’t automatically assume that a difficulty in the text requires conjectural emendation. It’s a matter of editorial judgment in each case, and reasonable minds can differ. It’s also relevant that the textual traditions of Greek drama in general, and that of Aeschylus in particular, are in a deplorable state of disrepair.
Page knew Greek as well as anyone, and he had a very acute intellect, though maybe his self-confidence sometimes led him astray.