Oh wait ... if only we had a university system for a few thousand years before student debt, that might provide insight into an alternative ! Oh wait ! We do !
Problems student debt tried to solve:
1) these systems do not let many people learn at all. Maybe 1-2% of the population gets higher education.
2) there is a large difference between institutions. And society's classes arrange around specific institutions one way or the other (price, geography, ...) (by which I mean the rich go to one school, the poor to another, with of course vastly different prospects). The institutions attended by the wealthy get the best professors, the best reputation, and their alumni get an advantage in the workplace.
(and let's just ignore that this is exactly the part that specific countries saved, and kept separately from whatever funding system they otherwise use for education: the part that worked best for the rich, like France's Ecole Nationale d'Administration, London School of Economics, and Germany's Bundesbank Hochschule. That's just peachy. Most of these schools you can only be referred into, you can't apply (or can't apply successfully), and they lead directly to the government's upper echelons)
3) There were much, much less study directions. Essentially anything you couldn't convince your parents to fund you learning for 2 or 4 years for could not sustainably be a university department. This resulted in much smaller arts, social studies, philosophy, archaeology, language, ... departments. Many people want to study those directions, but few feel like funding them.
4) Universities had perpetual funding problems, because everybody feels like regulating education. So the growth of administrators actually started before student loans were started. Whilst I don't agree with most idiots putting up this argument, there is a grain of truth to the idea that regulation forces universities into many unnecessary costs.
5) even the richest university directions, like economics departments in huge cities, weren't exactly swimming in cash. Therefore they focus on what brings in the bucks (more or less outsourced R&D for huge companies, creating spinoffs and some recruitment) ...
6) unwillingness of these departments to grow to the size people would like, and being pretty selective as a result (famously civil engineering and of course medicine)
Every proposal here essentially boils down to forcing education to be a reasonable investment, which will re-cause much of the above problems. If we're willing to do that, sure, but ... I don't think so.