I understood from the start that you are using your iPad as display for your Mac (that is what "Sidecar" is about).
I asked about using the cable, which now I know you are. So far so good.
I also know that "Universal Control" is a different feature, whereas your Mac and your iPad are each showing their native interfaces, but you can use the keyboard and trackpad to control them both.
Your question implies that, without you doing anything for it, all of a sudden you are taken out of Sidecar and into Universal Control (UC, for short).
Now I am a bit confused by your last comment. Of course you cannot do work on the iPad while also using it as display, but these are two different features, and you may want to use them both (just not both at the same time), so why would this be a silly function?! Say you have to type something on your iPad? It is far more practical to type on an actual physical keyboard, and you have your mac right there... so UC will be great then.
I agree that UC is not something that a lot of people use most of the time, and I imagine that most people may not even know about it, but silly I don't think it is.
Having UC activate when you are explicitly using a different functionality is a nuisance, yes, and we have yet to determine why and how it is happening, so I am not dismissing your problem, just confused at UC being called "silly"...