The US (not sure about Canadian) mobile market is completely different to the European one. It's almost alien in comparison - the carrier tries to control absolutely everything, and generally sees the handset as an extension of their network to the extent they want to "type approve" every firmware release on every handset that is "allowed" onto their network.
Apple appeared to become the exception due to having such an "in-demand" handset (going exclusive with AT&T initially might have helped them negotiate better terms with other carriers to get the iPhone to their networks) - mobile operators can't modify the iOS firmware at all. On any other phone, expect branding, pre-installed bloatware apps, modified startup and shutdown screens, modified top-bar to show "5G" on presence of a 4G network, and 6+ month delays to security updates while they get the above modifications made to the firmware before they approve it.
Arguably part of the "control" mindset comes from CDMA networks, where you couldn't "pop the SIM" and change network - you had to get the ESN whitelisted on the network, and that gave the carrier a lot of control over your ability to switch phone, or whether you could buy it from anywhere except themselves. I guess GSM carriers admired this level of customer-capture, and wanted to replicate it.
It sounds like "GSM" carriers are going to have to move fully towards whitelisting to preserve voice calling service when they get rid of their circuit switched fallback networks.
Inbound roaming of foreign users will be interesting in the next few years - without any CSFB voice support, they will likely not be able to serve most incoming roamers (who don't have network-to-network arrangement for VoLTE roaming), and that might cause a loss of roaming revenues once travel levels return towards more of a normal.