So they're incentivized to inflate their graduation rates to sell off the ISAs to investors who don't know better, since they need the cash now. Investors are left with a bill of goods: ISAs that will never pay out. Add in some securitization and tranches and baby, you've got a 2008 financial crisis goin'!
At this particular point in time, I suggest this is a vote of confidence.
This is a new asset class, investors are going to dig deep and understand it. They will certainly have access to more data and analysis than the average student.
However, if the government were to start to guarantee the loans and the volume hit an impossible level, and people got used to minimal defaults for 20 years then that would lay the groundwork for the type of disaster you suggest.
(I am not affiliated with labda or any of its investors)