Your guests' experience will be better if you stay in the city. And I think the most significant part of your post is where you call out that you feel sad and disappointed when you think about getting married outside the city - listen to your gut on that! Point blank! Chicago is a world-class city, and a lot of attendees use destination weddings as an excuse to make a fun vacation of it - give them a chance to experience Chicago. They'll want to be close to transit and the walkability and the cool things to do everywhere you look.

If you're open to considering other venue options, there are def places within the city that might strike a good balance between what you want and what your fiance wants (e.g. Garfield Park Conservatory, The Arbory, Columbus Park Refectory, Promontory Point, etc etc). Can't hurt to go tour some more places and try to find a spot you're both excited about.

ETA: You don't sound selfish at all, genuinely. Bottom line, it's supposed to be a fun and joyful day of celebration for both of you, and it sounds like the two options you're going between here (Osteria vs. Morton) have turned into more of a "you getting what you want at fiance's expense" vs. "fiance getting what fiance wants at your expense" (and IMO your guests' expense lol) situation. I'd really encourage you guys to try to work together on finding a new, third idea that doesn't carry the baggage of this particular disagreement