A Sequel is a brand new game.
An Expansion is something that just expands the core game, no matter if its plot starts mid-game or after the main game.
Having to load an earlier save is a bad design decision for games with an ending. Hopefully the following expansions will be post-end, not only because it is less annoying but because otherwise the rest of the game will become way too easy if you finished all the expansion content.
In this case the Devs would add few more Companions.
Only 8.6% of all Steam PoE users have completed the game?
Seems like that's pretty much a more than justifiable reason.
As someone who likes checking Global Achievement stats for games, I have already seen how surprisingly low percentages for "Finished the Game" achievements are.