Hopefully universities would find a way to "creatively adjust" their accounting such that graduate students are paid the same net amount as they were, but universities are big bureaucratic institutions and slow to change or adapt.
I'll only speak to what I know, which is fully-funded PhD programs in STEM. If this change hit students directly, it would be devastating.
If you look at top private institutions in the USA, you might notice that the socioeconomics of the undergrads skew rich-rich-rich but the STEM PhD students come from all walks of life (and countries all over the world). That's because the programs pay them enough to live on, albeit barely in many cases. If that fact goes away, so will the students.
Now maybe universities will be able to adapt and either pay enough to make up for it, or change their accounting. But most likely there will be a dichotomy where the richest and most successful programs have the easiest time negotiating the change while the others get hit hard.