If you follow the thread of what I said: I think even before Avengers they were thinking shared universe. Maybe not at the forefront but in the back of their minds they were thinking "This could be the start" and then of course after the Avengers they thought or at least we're open to admitting "this is for sure the start"
They've been trying to get a shared universe for how long? Since that old Batman vs Superman script in 02.(speaking of which I read it only once and didn't like it but I should dust it off and see)
Hell they were talking shared universe with GL before that went left.
They've been interested in the concept since before the MCU even formed.
People really think they weren't thinking shared universe back with MoS