Yes a reunion was pretty much guaranteed since it's SNSD, but a new album, music show promotions, many variety show appearances etc.? That was not, and this was pretty much what an active girl group would do for a comeback.
From what the girls have said in Soshi TamTam, their was no set year for this comeback. They tried last year but pushed it back to this year. Unless you mean SM planned it years ahead of time?
As for the second paragraph this is why I wanted to have this discussion, I wonder what was agreed upon for this comeback (royalties, payment for performances and appearances, etc.). Like the girls signed contracts for only this comeback, so this will become more common for veteran top girl groups? That would mean members can safely choose to leave a label for their career and it's not the end of their involvement with the group.
The first few years it didn't look like there was a clear plan but the last 2-3 years it was obvious a 15th comeback was happening. I think they said as much last year during the Yoo quiz thing. As for the extent of the comeback, I think has more to do with which members ended up staying.
I have a hate-love relationship with how SNSD paved the way for groups not clearly disbanding but I really do hope it leads to more reunions and comebacks