Option 1 is different from trying to cure someone from a regular incurable disease. The show failed to address any details on the Giftia technology, so it´s reasonable to think there there might be an opportunity to do something. They´re highly evolved machines, robots, androids, artificially created by humans, so there should be more control over their well-being than over that of the biological human body´s.
As for option 2, yeah, you´re absolutely right. But it still would have been more interesting to watch, and also more realistic. It´s totally different to cope with a regular death, be it via accident or of age, compared to a Giftia who looks absolutely fine and only has to die because some engineer made it that way. It´s unfathomably more difficult to accept. However, you´re wrong when you´re assuming this version´s Tsukasa would have simply sent a deadly Isla out into the wild. I´d expect a scenario where he keeps her locked up in a safe room. He´d then have a crazy relationship with a mad-gone robot girl, growing more fucked up with every day. There´s two routes from here: Either he discovers that Giftias actually go back to normal after some time passes, something that either hadn´t been tested before or was kept secret for unknown reasons, OR he´d eventually get to understand that his beloved Isla is long gone, that what he did to her was wrong, very wrong, and that he has to undo his wrong by finally accepting reality. Followed by some self-imposed punishment, like spending the rest of his life to better the Giftias´ situation. Maybe even becoming an engineer himself. Whatever.
The point is, all of that would have been more exciting to watch. What we got was some super vanilla pseudo-happy ending without any high or low points. It could be questioned why this anime even exists haha.