Mild necro, because this is something that matters a fair bit and I don't think we've had a conclusive answer either way?
Many now have monitors with high resolutions, but the non-scaling nature of the interfaces of previous PDX games means its almost impossible to use native res. I have to use a lower resolution when I run EU4 for example to ensure GUI elements are big enough. This prevents me from using borderless or windowed modes and mess up/move content on my two other monitors temporarily because of the resolution change. Its annoying.
In this day and age its standard to have a fully scalable UI. At A BARE MINIMUM I think there should be two UI modes; standard and oversized, the latter designed for 1440p/4k or people with 1080p who are going blind I guess. While Clauswitz may not yet handle fully scalable (though I hope it does) - I cannot fathom a single technical reason why two fixed UI size modes could be possible.
And while we're at it, bonus question, if its possible for HOI4, is it possible to backport whatever makes it happen to EU4?