1. Taxpayers already pay around 10,000 per year K-12 for a child's education. We should be doing more so that the bottom half of the population can get a job with that credential instead of pushing them into higher education - whether through apprenticeships, vocational training or ending preference for a degree when the job doesn't require it.
2. College degrees as a credential to show intelligence discriminate against the poor in a way IQ tests didn't, and it also punishes the working adults who have experience but don't have (and don't really need) a degree.
3. When you push people who aren't in the upper 50% of IQ into school, they end up with the easy degrees like liberal arts and social justice. Half the college protests and too many riots are people who earned a useless degree but expect a high paying job from it - if college is free, you'll get more activists with no valuable contribution to society.
4. Too many colleges aren't educating people in critical thinking or skills valued in the workplace but focus on indoctrination. Putting more people through that is bad for society.
5. If college is free while the bottom half of the population lacks the IQ to do it, they'll still drop out - and we end up with taxpayers subsidizing the higher income people who get the degree. This is the same problem with student loan forgiveness mostly benefiting the upper echelons.
6. We only have so much money, and we'd get more benefits from English second language for immigrants training and improving inner city schools than "everyone to college".