It depends on who you're referring to as a casual user. My mother for example would certainly have a hard time with it, then figuring out the key to bring up the boot menu (and being faced with a scary dialog that they've never seen), then selecting the right device, then likely being faced with GRUB which would also look scary to her, and by then she'd be overwhelmed before even getting to the install portion.

Yeah it is. Most computers come with windows pre-installed so most people never do this kind of thing.

And there's also things people need to be careful of. Like wiping all out all of their cherished photos by formatting the entire drive. Considering that casual users probably shouldn't attempt to do this. Not trying to gatekeep or anything, but there is potential for data loss for a user that doesn't back up their data properly, which is common for casual users.