Will this be the default?

Years ago (when I was involved in some QA work on the RT kernel), I did run it on my desktop. The desktop experience was generally more snappy and responsive under heavier load, typically the mouse and the graphical stack (when given RT priorities) did not have the same amount of lag.

Doing audio and video will also help a lot when those applications have RT scheduling. I would presume gaming could definitely benefit from it as well.

That said, if you enable RT scheduling on each running process .... the benefit will not be visible - in fact it can do more harm to the experienced performance.

I would expect that the next 5-10 years we will see kernel-rt being more commonly available as default in ordinary desktop environments with some performance profiles (managed by tuned-adm and similar tools). And it can definitely help those doing lots of multi-media work. MacOS is known for being quite reliable for such tasks and has for many years. The realtime kernel can definitely help closing that gap, at least to some degree.

Of course, lots of other things has also happened in the Linux world over the last 15-20 years (like Wayland, Pipewire, etc) also improving the desktop experience, as well as better hardware (moving from HDD to SSD to nvme). But I would expect that kernel-rt can be the icing on that improved experience, when properly tuned for the task at hand.