No - I challenge that, needs context
"It is not a good choice for a GUI application today, being Windows-only and a weak option even on Windows,"
That's bullshit without more context.
For developing custom applications used by one company with a limited user base yet complex business requirements, it's still ace. I can use a visual designer, I can code just on the back of the form, or I can use business classes and layers of architecture.
Where is the weakness here?
I did try Wpf but Microsoft wobbled, messed around and left me with the impression it was going away. It hasn't but when it takes a lot of time and effort to shift a project from one UI to another, why would I change if I think they're gonna chop and change.
I disagree with the whole "must throw away and jump on the latest thing" mindset. For businesses they need stuff to be stable, predictable and relatively easy to maintain.
Having constant new UI technology doesn't deliver that.
I like the idea that I may be working with a refined winforms technology in .Net 6. I'd loathe the idea of being told I must change to a different one, when for what we need, this works.