It may not be common, but I occasionally run into apps that will work will with limited resources but that will happily expand outwards if given the opportunity. Perhaps a bad example, but the code for this cryptominer itself checks how many CPUs you have before it starts.

You are often right that you either supply the resources an app needs or you don't. However, there are a growing number of apps I'm seeing that act more like goldfish - they grow to the size of the container they are put in.

I also occasionally run into apps where I'm OK with bad performance, I just don't want them to interfere with other tasks that I have.

I might decide that I'm OK with a version of Slack on my work computer that runs poorly and that occasionally starts caching stuff to disk - as long as the rest of my computer doesn't slow down. Not every app that I'm using needs good performance - some are more important than others. This is especially true for background apps like a backup system, file sync, update, anything where I don't really care if a task takes longer to finish.

It also might be worth exposing some kind of more fine-grained policy; something like "I want this app to have full access to my CPU if it's in the focus, but if I minimize it, I want you to reduce its resources or even suspend it."

And of course there is the (perhaps naive) hope that as CPU and RAM become a resource where users control access in the same way that they control location or camera access, developers might start to include resource-heavy features as progressive enhancements. This has... sort of... worked on the web with resources like location. So it's unlikely, but possible.