So hey, you take a selfie, 3 MB. And now you want your selfie to show yourself posing in front of the Eiffel Tower, the Statue of Liberty, the Great Wall of China, and the Grand Canyon. OK, now you've added an additional 12 MB.
Do that often enough, and you'll have to buy another block of Google One storage, which is MRR for Google.
Google photos offers a save as copy / save as original feature when editing photos and videos. Removing the save as original button would be cheaper and significantly more effective.
I think it's more likely that they're doing it because they think it makes their product better, increasing sales that way instead.
Every product at Google is being pressured intensely to put AI everywhere. The focus is on velocity, not well thought out product experiences. This sort of photo editing interaction is not novel so it doesn't require a ton of out of the box thinking to decide to add it to Photos.
Google is worried that other companies are going to steal their lunch and their approach is to try to throw every AI feature they can into their products as fast as possible.