I am assuming means testing is a political call. Polling on the topic in very broad strokes is pretty supportive of some degree of loan forgiveness but that support dries up a lot without means testing. This is a baldly political decision (IMO) by Biden here so I'm not surprised he's aiming for that.
I am wondering what the angle taken on an SC challenge would be. The federal government guaranteed the loans for ages, went direct in the 90s, and there is a long history of loan forgiveness. In 2019 Donald Trump forgave all student loan debt for disabled veterans via an EO using the same law that
Biden is likely using to support this EO. There has also been various over loan forgiveness EOs, mostly to students who went to for-profit or fraudulent schools. This isn't to say a lawsuit won't come and it won't reach SCOTUS, but the cat may be out of the bag and there is precedent to this - just not at the scale Biden is doing it.