I'm not a developer, but I can tell you for a fact, it wouldn't take a day or two. You have to replace all Redanian, Witch Hunter soldiers, things, banners, posters on the wall, everything connected to the Eternal Fire cult, change dialogues, make new dialogues for folks, connect everything to the situation, check if eveything works. That's a lot of work for one of the biggest and the most 'dense' cities in gaming. And that's just one thing.
NG+ was demanded by so many people - so CDPR added it, later in the development. I'm not bothered by it at all, because for me it's impossible to play after the ending anyway.
Maybe it was, but there was a huge demand from people, so they added it. You don't have to play afterwards.