The decline of the rust belt wasn't a policy issue. US auto manufacturers failed utterly when the time came to shift from an innovation market to a consumables market. No policy by any government anywhere could have saved them.
The Rustbelt was an incompetence issue as a side effect of other policies. Reagan began several anti-jobs policies when he took office. The Rustbelt as we know it is the outcome of them.
Policy 1: Destroy labor. By making it clear that the power of organized labor was dead, and that labor would not be respected, Reagan made it impossible for labor to fight to keep jobs in union states. Companies faced no consequences from moving jobs to union busting locations and even overseas. In fact, American companies get a tax break for doing so. The taxpayer pays companies to send American jobs overseas.
Policy 2: The design of the Reagan tax cuts discouraged domestic investment. Individual investors simply had no incentive to invest in companies that invested in the US.
Policy 3: Acceptance of higher average unemployment rates. This both harmed investment, and made sure labor got lower wages.
Policy 5: Deregulation of finance. It became more profitable to speculate than to invest. It became more profitable to do mergers than to grow companies.
Policy 6: Cuts to domestic public investment. Less infrastructure spending means less jobs in many industries. Particularly American heavy industries.
Policy 7: Cut back the welfare state, so that labor is even more desperate, and won't fight back.
Policy 8: Massive deficits: Foreign exchange rates respond to monetary and fiscal policy. In the 70s American manufacturers first faced serious competition. Now they didn't do well for a number of reasons. But neither were they going to ruin. Reagan's deficits made the difference to make them go to ruin.
It caused massive changes in the foreign exchange rate of the US dollar. And that made American manufactures much less competitive with foreign manufactured goods.
Yes, there were trends which have been against American companies for a long time. But Reagan really accelerated all of that.
I'm surprised to see you of all people complaining about manufacturing jobs being sent overseas. What should he have done to protect them?
I'm not opposed to foreign trade. Nor am I for protectionism. But in order for trade to benefit everyone in a country policies still have to have the importing country working to keep it's economy as competitive, dynamic, and high investment as possible.
Developed economies are going to lose low value added manufacturing jobs. Nothing can be done about that. And high value added manufacture is going to require fewer workers for the output. Productivity goes up, and nothing can change that. This is natural. That said, there is nothing which prevents, or in any way restricts, high wage nations from having high value added manufacturing jobs. Germany and Japan are not low wage nations. Yet they do well with manufacturing. Why? Because productivity is high. But that high productivity requires high investment. And "Supply side economics" is anti-investment.
Conservatives are so utterly determined to redistribute wealth that they completely destroy the creation of wealth.