Unlocked bootloaders everywhere would be nice but there are already manufacturers that give you that across the range. The big issue is that none of the drivers are upstream so even with LineageOS you're stuck on and old and often unpatched kernel. When a new Android version comes around it's common for support to be dropped for devices. Right now there's no Ubuntu-like experience with Android. Get a Pixel or an Android One phone if you care at all about security and access to new Android versions.