I wonder if this was at all partially retconned from what Feige had already envisioned/planned for the Inhumans.
Inhumans sounded like it was gonna be his baby as he was a big fan of the group. Until he sacrificed them by handing them over to the TV division to prove they weren't worthy of being part of the actual MCU. And boy, was he right..
I wouldn't say they weren't worthy, more like there were things specific to their makeup that he saw as problematic: similarities in their characters/story lines with other properties (like Maximus as the evil genius brother of the king who's always scheming to take the throne - i.e., a poor man's Loki by virtue of being introduced after him). Probably the biggest problem was that THE lead role (Black Bolt) is essentially a non-speaking role, so good luck landing a charismatic A-Lister to play him. Feige would have had his pick of A-List actresses to play Medusa & I'm sure they could have filled the roles of Gorgon, Triton, Karnak & Crystal with top talent as well (and probably made Lockjaw another Groot/Grogu), but without a great Black Bolt as the anchor Feige could no doubt see the limitations. Plus by then he may have been growing more confident about getting the X-Men back.