It's not really the same when the school gets the money either way and it's not clear how the placement rates really translate to good outcomes (it puts the rationality burden on the students which is the wrong place for it and less likely to work for the reasons you describe). Schools are incentivized to not have students fail out and to select hard for students already likely to succeed during admissions, but that doesn't mean the students currently learn much from the school or that the schools should even make education their focus (as opposed to increasing their prestige via research or facilities to attract more students).
If student failure to succeed in the market led to university failure we'd see a lot different behavior from universities.
I don't think the universities are currently selling education, they're selling prestige and network access via credentials with some education on the side at inflated rates from easy access to government non-defaultable loans.
This leads to weird behavior like all majors being the same cost and the same length along with lots of people employed by the university that don't know anything (career services where I went to school was a good example of this). There's also a lot of spending on facilities to lure students that is unrelated to education or success afterwards (since they have to compete with other universities for all of the free government loan money).
If this free money was not available there would be a check on people getting loans for majors that are not economically viable. If school's offered ISAs then the school's would provide the check. As it is currently, the student carries the burden of evaluating all of this - it's not a surprise a lot of people fail this at 16.
This incentive alignment issue is why people come out of universities and get destroyed by technical interviews, why a large amount of CS majors can't solve fizzbuzz and why people pay a lot for ultimately worthless degrees. I don't disagree with the cultural problems you describe, but I think if there's an ISA option vs. 100k in debt option more people will take the ISA option if the education actually works (and it will have to for ISA providers to survive).
There's a way to fix the systemic incentive issues and place the burden of evaluating economic viability on systems better able to handle it. This would fix a lot of the problems, but it's hard to change from within the system with the current broken incentive structure. That's why I'm excited about the Lambda School approach - I don't think universities can dig themselves out of this and I don't think the necessary changes to government loans is politically viable.
This is also why I think 'free college' is bad policy, it further prevents Lambda School like ISA approaches and cements in the current broken incentive structure.