What would be wrong with open borders?

The U.S. needs an "open borders" policy. This need not mean changing the existing law about various categories of migrants to be admitted. Rather, it should mean creating a new category of migrants admitted with minimum restriction or processing and no tax-paid benefits to them. The no-benefits rule must be strictly enforced. The main rule for this new category of migrants is that there is absolutely no tax-paid benefit, and there is a fast-track procedure of admitting them quickly. 

Many of the existing migrants would choose this option, but they would receive no public assistance of any kind. This would prevent much current cost and waste and make travel easier. And these new migrants would join the U.S. labor force to the benefit of U.S. consumers, by helping fill jobs which are currently open and thus causing the production level to be lower than desirable.

Arguments against such an "open borders" policy: 

Migrants would overuse the public schools, drive up costs.

solution: The "open borders" policy must contain a qualifier allowing states to refuse to accept in the schools those immigrants who entered under this policy. They would be free to limit their acceptance of them, or accept however many they choose and no more. Or none at all.

States can address this differently as they choose. There should be no federal requirement that states must accept these immigrants into the tax-paid schools.

Migrants would overuse free health services, crowding into hospital ERs.

solution: The "open borders" policy must contain a qualifier allowing hospitals and jurisdictions to exclude the open-borders immigrants from accessing free services. The providers would be free to limit their acceptance of them, or accept however many they choose and no more. Or none at all.

Migrants would abuse public assistance programs, driving up costs.

solution: The "open borders" policy must contain a qualifier excluding these migrants from public assistance programs. States or jurisdictions could choose to extend such services to these migrants, at their expense, but all federal programs would be closed to these migrants, and states and jurisdictions could adopt their own rules for their own programs not supported by federal tax dollars.

The rules excluding open-borders migrants from public assistance must be strictly enforced. This exclusion rule might be limited to a period, like 10 years, after which the migrants would become eligible to apply for public assistance. 

Migrants would steal jobs from American workers.

solution: American workers must stop whining and learn to compete in the market. The purpose of their jobs is to serve consumers, not provide them with a guaranteed income regardless of their performance. Wage-earners should have to compete, just as the independent contractors should. More competition is always good for consumers = population of 330 million.

Migrants would bring contagious diseases into our country.

solution: Migrants entering under the "open borders" policy must be required to pass a health examination, which would exclude any who have a contagious disease. This should be the only barrier to entry. All those cleared by the examiners would be allowed entry into the U.S. A fee should be imposed, $100 or $200 per migrant, to pay the cost of this exam.

Migrants cost us too much, for the holding facilities, the hearings, the processing, etc.

solution: There need be no holding facilities for the migrants who enter under the "open borders" policy. And no tax cost in processing them, and no need to detain them. They should be admitted immediately after passing their health exam. No hearings would be necessary for these migrants, as all are entitled to admittance if they pass the health exam. So there's nothing to hold hearings about. No need for high-paid lawyers and judges.