Trump did literally nothing to save the plant. Others could have provided incentives. Remember it was the US government that bailed out GM with 50 billion taxpayer dollars, losing $10 billion.
The sad truth, however, is that in the end the real power to stay or go is not a public decision but a private/corporate one. GM holds all the cards — like all corporations do on economic decisions. There was a time when tax abatements were tied to retaining or increasing jobs, but communities have largely lost the power they once had due to corporations being able to play one community/state/nation against another.
Attaining political democracy can only go so far without movement toward economic democracy. p.s. Of course, GM stock soared on the announcement.