Large libraries (e.g., 200GB+) may take significant time to update project media upon the first launch after installation. Apple Discussions troubleshooting a specific error you're seeing while trying to install this version? iMovie - App Store - Apple
Sometimes, "new" doesn't mean "better" for everyone. Major updates can change the user interface or introduce new bugs that disrupt workflow. Many video editors prefer to stick with a version of software they know is stable. If you started a project in 10.2.5 and it crashes in 10.3, reverting to the older version is a necessary troubleshooting step. Download Imovie 10.2.5
If you have previously "purchased" (even for free) iMovie on your Apple ID, follow these steps to trigger the download for 10.2.5 on a compatible system: Large libraries (e