You don't think they will keep the Delve?

It's already part of the core game so it's here to stay. Players also really love it!

They just once in a while have a special 1 month long league/event where they brining back the "full" version of past leagues.

Often times there where special item drops that are not included along with other balancing changes made to be able to intergrade a league into the core game.

During those special leagues drop rate tend to also be a lot higher to make up for them being much shorter in time. They can get quite crazy and fun.