Geiranger being a popular port but with limited capacity is snapped up early in the process by cruise lines. Disney doesn't get its Europeans itineraries together until late in the game - which forces it to take whichever ports are available.
British Isles are very easy to do on your own if you have a few days to spare before or after your cruise. In particular, you can spend more time in Scotland and visit the likes of Isle of Skye, Loch Ness, Eilean Donan Castle, and Scottish highlands. You can't get to them all with just one day in Greenock. On the flip side, the cruise will save you the ferrying part of a DIY trip to/from Ireland.
There is a chance Disney might send two ships over to Europe in the summer in 2022 (or later). That will open up more itineraries - possibly longer Norway itineraries. If you can, put a deposit on both 2021 cruises. Wait for 2022 European itineraries to come out and then decide what you want to do.
Ps. We were on Disney's 2019 cruise to Norway which stopped in Geiranger.