As the Star Wars sequel trilogy released, theories surrounding returning characters were at an all-time high. Anything seemed possible, from Hayden Christensen returning as Anakin to Ewan McGregor returning to Obi-Wan as a Force ghost. In the end, neither of these characters made an appearance. However, director Rian Johnson admitted that he wanted Obi-Wan to return in Star Wars: The Last Jedi but wasn't sure if Ewan McGregor's version would fit.
There were once many theories surrounding Rey being Obi-Wan's daughter, and while those never came true, it was still possible to fit him into the trilogy. With Luke alone on the planet Ahch-To, he could've easily been visited by a Force ghost version of Obi-Wan, alongside the return of Yoda. And Johnson was well aware of this and definitely considered it.
During an interview with The Playlist, Johnson spoke about the matter, revealing, "I would have loved to have had Ewan McGregor in the movie but it was just a matter of story-telling." He then explained the complications of using McGregor, saying, "obviously, if Alec Guinness were still with us, that would have made sense. But we never saw Luke ever interact with the Ewan version of Obi-Wan."
While there are countless fans who would've loved to have seen Ewan return to his role (before his own series), Johnson does make a fair point. When watching from a chronological point of view, seeing the younger Obi-Wan again would be jarring, especially since Alec Guinness' version makes an appearance as a Force ghost at the end of Star Wars: Return of the Jedi. Johnson believed that because of this, there would be "less of the emotional connection, and it might have been a little odd."
However, Rogue One: A Star Wars Story released a year prior to The Last Jedi and featured CGI actors who had since passed away. A computer-generated Leia is revealed right at the end, and GCI Tarkin has a reasonably large part to play in the movie. But opinions on this were mixed, as while the technology is undoubtedly impressive, a certain uncanniness lingers.
It's also unknown if McGregor was made aware of his potential return or if he would even have taken the role if offered. He recently jumped at the idea of playing Obi-Wan in his own show, but for a while, he was seemingly done with the franchise. And with the constant discourse surrounding The Last Jedi, it's perhaps best that he wasn't in it, as he's already had his fill of controversial Star Wars projects.
In the end, regardless of the reception to The Last Jedi, most will agree that McGregor had no place in the story. His appearance would've likely stuck out as needless fan service, and the scene between Yoda and Luke remains a great moment on its own. And with the release of the Obi-Wan Kenobi show, fans got the return of McGregor anyway.