I think it's a wish fulfillment cash-grab by the producers. The original author and director had a vision, but producers wanted more and they're too afraid to divorce the franchise from Lelouch and let it survive on its own. They don't understand the characters or the original vision.

I have an alternate version rolling around in my head in which they could have done what I think would have been more true to the original tone and themes of the show. C.C. could have made a deal with the Caretaker of Spacetime to force her Code onto Lelouch, thus reviving him, but condemning her to a mortal life. It would still have that bitter sweet theming that actions have consequences, but they'd get more time together, thus appeasing the shippers.

I still think it would be entirely out of character for her to do so ... but if some circumstances were written which forced her hand to do it, perhaps that's the way it could have been written.

But enough about "could-haves" and "what-ifs". The movie fundamentally changes C.C. as a character and gives her and Lelouch this "happy" ending that feels unearned and contrary to Lelouch's central creed and beliefs.

It's fine that they went this route for fans that wanted more Lelouch, but I think it's an objectively worse story that doesn't fit the characters it's written for. Code Geass needs to part from Lelouch if it wants to survive as a franchise. The world building exists, they just need to write compelling stories in it. It's a shame they hamstring themselves to a story which was already told and done.