It was announced when Perlmutter was still in power. Most likely a longer time before we even got that famous event back then since they dont decide this from one day to the other.
I think the major difference between those projects and Inhumans was demand.
Avengers is definitely going to be a thing for Phase 3. But Captain Marvel has been something fans have wanted for some time, along with pressure to have a woman in a leading role.
I could be wrong, but the Inhuamns project is that left field choice. No one hates it, but no one really asked or expected it either.