Considering that they can't manage to not break foundational libraries in code paths that should require no changes (at least since the introduction of Aqua, with the actual code being used since 1988!) and considering how they broke even 64bit software in Catalina in subtle ways that are still impossible to figure out by developers...

Nah. It won't be seamless at all. It will be like touchbar macs - you buy it because last 3 generations were stuck in place with little improvement and you are forced artificialy to upgrade.